It's fairly difficult to say, because the people creating the records we would use to determine what was going on with nineteenth-century prostitution - the middle classes, largely - understood "prostitution" as a much broader thing than we do today. It did not just mean the exchange of a set amount of money for sexual intercourse, but a sort of more general catch-all for poor women having sex outside of marriage. When opponents of birth control during the period feared that women would become "prostitutes" if not forced to consider pregnancy (and the need for children to be legitimate in the eyes of the law and church, and the need to support them financially) when having sex, they weren't worried that women would take up sex work to earn money, but that they would engage in casual or premarital sex. This is highlighted extremely well in the recent book by Hallie Rubenhold, The Five, about the five canonical victims of Jack the Ripper. All were dismissed at the time as prostitutes and are commonly thought of that way even now, but her research shows that only Mary Jane Kelly was literally putting a price on her bed: the others were simply in the position of needing to be in a long-term monogamous relationship with a man in order to afford food and lodgings, or to be protected when sleeping rough. Is that transactional? Yes, but the same could be said of marriage at all levels of society at the time, given the fact that women's occupations were poorly compensated and that young women of the middle and upper classes were not trained to do paid work in the first place, in general. This sort of relationship was common in the lower working classes, as I discussed in this answer to a question about teenage romances:

Among the poor, sexual relationships appear to have been similar in that they were serious rather than casual, but they were as likely to occur from economic necessity as from preparing for marriage: the changes caused by the Industrial Revolution made it harder for a single woman to support herself, so cohabiting with a man outside of marriage was seen as a possible way of getting a place to live and food to eat while earning pennies. However, this could be dangerous - if she became pregnant and he abandoned her, she was worse off than before, and if her birth family couldn't afford to take her back in with or without her child, she would end up in the workhouse or living on the streets. But serially monogamous sexual relationships were common, essentially like marriages followed by divorces - but in a period where divorce was next to impossible for the poor to achieve, it was simpler to not marry in the first place. (That being said, a number did marry-for-real and simply bigamously remarry if deserted by a spouse.)

That being said, there were women who were unambiguously sex workers. For many of them, it was temporary: something to be done when times were very hard and there were no other options. These women were often born rather poor, the daughters of manual laborers and craftsmen, and many of them began on the death of their fathers, the primary wage-earners of the family. (This kind of precariousness is also well-illustrated in The Five, by the way.) They might alternate sex work with a few days or weeks in the workhouse, particularly if they fell ill - it offered very little independence and endless hard work, but also guaranteed you a set number of meals and a bed for the night. Begging and theft were used alongside prostitution to pin together a meager living when nothing else was available; siblings or extended families might be able to offer money or a place to stay that would allow women to try something else. For some, the cycle was unable to be exited, but others would marry out of it or get work in lower-working-class professions where employers wouldn't care very much about their backgrounds (working in a pub, sewing, manufacturing of some kind).

However, the entire class of Victorian prostitutes was not made up of these women, who are somewhat overrepresented in the records made through workhouses and prisons because it was the poorest who would end up in them. Many others did this work because it paid significantly better than the jobs available to women. Sex workers also seem to have mainly catered to men of their own classes, so a bank clerk's daughter who "fell" would be earning the money a shopkeeper could afford to pay, in contrast with a destitute woman who depended on sailors and factory laborers. These women did tend to think of it as a career, planning out their beats and negotiating prices in order to get the best deal, as well as delineating specific time off (or choosing only to work a shift until they earned a specific amount of money). However, if they were from the middle classes, there was really no way to come back from it and transition into respectable life again - it had to be a career for them, even if they stopped actively sleeping with men and ran a brothel or acted as a maid for another prostitute instead. Getting work as a domestic servant or governess would be impossible without a reference, and extremely few respectable men would marry a woman they knew to have been a sex worker. (One exception, using the broader Victorian definition of "prostitution", is Charles Dickens's mistress, Ellen Ternan; their relationship was kept very secret during his lifetime, and after his death she presented herself as many years younger than she was, and married legitimately.)

A very few were not just earning a good living but mixing with the highest echelons of society - courtesans. Rather than being paid by the act and having to walk the streets to find customers, they would be "kept" in fine residences and given jewels, clothes, vehicles, and horses. Some managed to save their money and property for the time when their looks were no longer youthful enough to ensure customers; others ended up traveling back down the ranks into ordinary prostitution again, selling their possessions when necessary.

I cannot recommend The Five enough, but I'd also cite Common Prostitutes and Ordinary Citizens: Commercial Sex in London, 1885–1960 by Julie Laite as a resource for this post.