This is one of those basic factual questions that should be relatively easy to answer, but isn't. I have a few data points from different times, workshops, and styles of stone that suggest that the answer is, unsurprisingly, "it depends."
The records of the John Stevens Shop in Newport, RI provide some of the best information. During the 1730s, the Stevens shop charged £12-15 for a table-like "Tomb Stone," £1-3 for pairs of upright gravestones, and 2 pence per letter for inscriptions.
Thus, the heirs of Captain William Wanton paid £17, 5 shillings, and 6 pence for his elaborate tombstone on December 24, 1733 — £15 for the stone and the rest for 273 letters.* A year earlier, Samuel Lindon paid £3, 8 shillings, and 4 pence for a pair of gravestones with a 40-letter epitaph "for Priscilla."