It depends on which part of the 1700s, which country, and which painter, but here's a little bit of information that I happened to have on hand:
In 1780, the price for a half-length portrait from the top British portraitists:
(source: Paula Byrne, Perdita: The Literary, Theatrical, Scandalous Life of Mary Robinson. New York: Random House, 2004, p. 154 [paperback version]. Each of these painters did portraits of Robinson. Byrne's source is unclear; it may have been the artists' records, newspapers of the period, or Mrs. Robinson and Her Portraits, 1978, by John Ingamells, all of which she cites in her book.)
A guinea in 1780 was valued at 21 shillings, so using this currency converter from the National Archives of the UK, I calculate that a half-length portrait by Reynolds would be £4,520. This gives a suggestion of what else could be bought with that amount of money at the time.
Meanwhile, in Colonial America about 1771, John Singleton Copley, an up-and-coming artist, was charging his Boston patrons "between 11 and 15 guineas for oil portraits, depending upon size and, one suspects, the sitter's wealth," according to author Alfred Frankenstein.
In 1771, Copley went on a working trip to New York, and wrote in a letter that his portrait prices would be higher:
"Weither Men or Weomen makes no differenc[e] in the pric[e] nor does the Dress, but Children in the 1/4 peaces will be more, because of the addition of hands, which there must be when a Child is put in that size; but should the hands be omitted...the price will be the same as for a Mans or Womans without hands." [Original spelling retained.]
According to the author, Copley's instincts about New York were correct, and he had no trouble finding clients there at these prices, partly because of a lack of competition.
(source: Alfred Frankenstein and the Editors of Time-Life Books, The World of Copley: 1738-1815. New York: Time-Life Books, 1970, pp. 64-66.)
Copley produced 1 or 2 portraits a month, but he tended to work slowly, according to Frankenstein. Some painters, such as Reynolds, had many assistants, and so could produce more.
Edited to remove a superfluous word and say thanks for the award!