I can give you a partial answer, for the Colonies.

Like anything manufactured in the 18th c., they were what we'd call expensive, because of all the labor required. The cost depended on what the gun was. George Washington order a very nice fowler from London gunsmith John Brazier in 1767. It had silver mounts ( i.e. triggerguard, buttplate, etc) a new-style water-resistant lock , a hook breech so the barrel could easily be removed for cleaning, and a case. That cost eight pounds and eight shillings. On the other hand, a trade gun, very basic , would go for about four pounds and sixteen shillings. An old musket might go for as little as three pounds. Pistols were not as useful and not as common, but often were sold in pairs. In York Co. in 1740 a Pennsylvania merchant ordered for resale from London a pair of simple "pocket pistols" ( maybe something with a screw-on barrel) for eight shillings, a pair of bigger ones ( that you sometimes see mistakenly called dueling pistols) for one pound fifteen shillings. At this time in the Colonies a carpenter might get around six shillings a day, a laborer two shillings. ( 1 pound = 20 shillings). Those wages would be higher than in England- (there was a labor shortage in the Colonies) and I don't know what those would be.

It can be tempting to assume that, based on the expense, not many people owned guns. But farmers did, and in the rural Colonies there were a lot of farmers. For England I honestly don't know, but am more dubious: wages were lower, and hunting was a more aristocratic pastime requiring the use of someone's estate, and poaching was severely punished.

Harold Gill, Jr : The Gunsmith in Colonial Virginia

History of Wages in the US

Bodark's answer is great for the American colonies but I'm going to go over to England.

So, you're a gentleman in 18th century England. That means you have an income of at least 200l. from lands you own. And even that's low, you might be more of a very wealthy yeoman at that point. Though legally there would be little difference, 200l. was plenty to fulfill all property requirements virtually everywhere, socially there would be a distinction. It would be rather blurrier in the countryside, especially the part of the country where you owned land, but if you went somewhere you were not known they might politely inquire about class markers. ("Which university did you attend?" with the proper answer being Oxford or Cambridge. Maybe that backwater Harvard if you were a colonial.)

You are not the average person. Even though you're barely scraping into the lowest class of the aristocracy, you're much wealthier than even a master tradesman, who would make (if they were average) about 50l. per year. And you're much, much wealthier than unskilled laborers, who would make about 10l. per year. So you can absolutely afford a gun.

How much would it cost you? Unlike the colonies, where gunsmiths still make the majority of rifles with a relatively high degree of customization, often with expensive imported material, things are much more standardized, local, and industrialized in England. There are actual gun manufactories in Birmingham and London that pump out hundreds of thousands of guns on standard patterns. (This will rise to millions in times of war.) One of these pattern muskets might cost as little as 15s. (3/4ths of a pound) new when prices are low, and less on the used market. (You'd probably have to pay extra for shipping or travel to a store.) Some more expensive models might rise to two or three times that, especially if it was something that had to be machined like a rifle. In times of war, even a colonial war, these prices will increase as demand for guns goes up due to both military and civilian demand. Sometimes dramatically. Otherwise, they'll fluctuate about 25% throughout the century.

Of course, if you want to pay a few pounds, you could get it made custom by a gunsmith. As a gentleman, you probably do. Owning custom, well made objects is part of your class. Buying something obviously cheap will be a little problematic when you go hunting with friends. And paradoxically, since you have such a small estate, people will be watching the quality of your gun more (with the usual gossip accompanying that watching). The price will then depend on exactly what you want but. Fortunately, most gentlemen spent at most a few pounds, so this is well within your budget.

Leaving your rarefied atmosphere, could an average Englishman buy a gun? Yes, if they were willing to stretch their finances even a poor laborer could buy a cheap gun. While wages were higher in the colonies, gun prices were much, much lower in England. (There was a brisk trade buying guns and gun parts and shipping them to the colonies.) That said, it wasn't likely most of your workers would have one.

There was less reason for an Englishman to buy one. Britain saw less invasions, rebellions, and general disturbances than the colonies. Hunting was also less legally supported and there were fewer dangerous animals. Colonists didn't buy guns because they thought they were neat, they bought them because they needed them to keep out pests, dangerous animals, and invading Frenchmen. These concerns were simply less present in England. (There were also some regulations on who could buy guns in England that were absent in the colonies, but these affected, at most, a fifth of all Englishmen. They might have had a small downward effect in England. They undoubtedly had a huge downward effect in Ireland.)

From The Birmingham Gun Trade, Gunpowder, Explosives and the State: A Technological History, and The English Gun Trade.

Thank you so much for these answers. Exactly what I was after!