By customer I mean people who smoked Opium in these opium dens. Could working class people afford to support their habit in the same way that people today can afford smoking cigarettes?
And what would the price be at such a place, in victorian era currency?
Opium would have been much more widely available as laudanum (opium dissolved in alcohol). The existence of opium dens in Victorian London has actually come under a lot of doubt as historians have researched it more; Steven Martin, author of two books on opium smoking, found the lack of photographs of any opium dens in London evidence that the entire idea of London being crawling with these opium dens was mostly a complete fabrication.
http://www.opiummuseum.com/index.pl?pics&67
In places where opium dens were more common, the price was obviously quite variable, depending on weather, yield, demand, and so on. On top of that, an individual daily user's daily amount could range anywhere from around 3 pipefuls a day, to 50 or more, and users' habits would shrink or expand depending on their own circumstances and the cost of opium, so it's very difficult to say, "Here's how much a 19th-century opium smoker spent."
But to give some idea, here's Zheng Yangwen.
Five candareens (a measure of weight) was the minimum for the novice smoker. This did not cost too much. Two to six copper cash could purchase five candareen of prepared Malwa in Wenchow. In Chefoo it was four cash, and in Canton it varied from four to nine cash for three to five candareens of prepared foreign opium. Four cash was the standard, according to the Maritime Customs and officials such as Xue Fucheng. Yunnan and Sichuan opium, when foreign opium disappeared, would also cost four cash. This demonstrates the popularity of western China opium, as other native opium would be as low as two cash. Cash are copper coins with a square hole cut in the centre through which they can be strung together. A string was normally ten cash. Although its value fluctuated depending on the locality and the time, it was the standard exchange for basic services, such as a rickshaw ride, a tip or a simple meal.
Yangwen, Zheng. The Social Life of Opium in China (Page 161). Cambridge University Press. Kindle Edition.
The weight described, five candareens, is about 1.5 to 2 grams. The majority of addicts used anywhere from that amount, up to 12 to 15 grams per day, with a very small minority using a very large amount, 30-50 grams or more. So the answer to your question becomes something like "Anywhere from half the cost of a meal, to the cost of 25 meals or more."