So, first of all, not every country propogated music in precisely the same way. London and Paris, for instance, had booming sheet music publishing industries throughout the century, whereas Italy relied pretty much entirely on copyists making handmade copies of musical manuscripts until very late in the century. Indeed, copyists were a pretty popular way of acquiring sheet music throughout the continent -- Jean Jacques Rousseau, for instance, was a highly active professional copyist in Paris. Of course, the work of a copyist is harder to keep track of as it isn't standardized or often advertised through catalogs with specific prices. Published sheet music was, though, so we have a pretty good idea of what that cost.

Just to make things easy for ourselves, let's say we are buying printed sheet music in England sometime in the 1760s or 1770s (In that case, if you are buying music by Bach, it is probably the work of Johann Christian rather than his father Johann Sebastian). Two popular music printers at this time were R. Bremner and Longman, [Lukey,] & Broderip, And we actually have copies of their catalogs to reference. Here's an R. Bremner catalog from 1762, and here's one from Longman, Lukey, & Broderip from 1774 (this one contains an advertisement for instruments at the end, though without pricing information). There's a whole repository of music publisher's catalogs from various epochs that you can access at IMSLP.

At the time, English currency was tripartitioned into pounds (£), shillings (s[h]), and pence (d), which account for the three columns of Longman, Lukey, & Broderip's catalog, though as you can see, no sheet music exceeded a pound. Perhaps to save space, Bremner's catalog only has shilling and pence columns. The way this currency worked, there were 12d to the shilling, and 20s (or 240d) to the pound.

According to this currency converter hosted by the National Archives in London, £1 in 1760 is equivalent to £102.46 today (well, actually 2017, but close enough), 1s = £5.12, and 1d = £0.43. We can use that to estimate the modern day cost of works from R. Bremner's catalog. In the following table, I take a work type, average the cost of all such works in Bremner's catalog, and then convert that into its 2017 equivalent.

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Work Type	Average cost in 1760	Equivalent in 2017
[Overtures] For Concerts^(1)	12s 11d	£66.17
Periodical Overtures^(1)	12s	£61.48
Italian Opera [Score / Parts]	7s 8d	£39.28
Italian Opera [Vocal Score]	4s 10d	£24.76
Trios	6s 7d	£33.73
Violin Duets	4s	£20.49
Violin Solos (w/ Bass)	5s 9d	£29.46
Flute Solos (w/ Bass)	3s 11d	£20.07
Flute Duets	2s 6d	£12.81
Flute Trios	5s	£25.62
Harpsichord Music	6s 7d	£33.73
Guitar Music	2s 7d	£13.23
Vocal Music	4s 4d	£22.20
Other Music	2s 11d	£14.94
Miscellaneous	1s 2d	£5.98
Treatises / Books	3s 4d^(2)	£17.08

^(1) I assume these are full sets of orchestral parts. Though that still seems absurdly high given that, say, a full set of opera parts is almost half the price. Not sure why these are so expensive.

^(2) In this category, all of the "instructions for all instruments" series are counted as one entry, since the individual instruments aren't listed. As these are 1s 6d each, if we counted each kind of instrument instruction book separately, it would probably drive the average of this category down significantly.