Over on the forum, somebody asked me how much frankincense and myrrh cost in antiquity. Well, we don't know. We never get as much price information as we like out of history. There's the occasional mention of this or that product being worth its weight in silver or gold, but we can reasonably believe that those are descriptions written by political, cultural, or philosophical commentators trying to give an idea of how expensive things are, not people who have actually done an economic analysis and discovered that the material in question is, by coincidence, exactly the same value as silver or gold and not, say, one and a half times the price of silver or three quarters of the price of gold. We know it was valuable, but not how valuable on the level of individual purchases.
But for the convenience of the GM, we can use those comparisons to establish some ballpark figures. Mind you, this is based largely on GURPS rather than historical sources, so it's not something I would have added to Incense Trail, but it might be a generally useful exercise. Frankincense and myrrh are incense (GURPS Low Tech, p. 36), with a base price of $6 to $15/oz., and it's emphatically subject to luxury pricing (Low Tech, p. 37). Silver is canonically $1000/lb. or $62.5 oz. Gold is canonically worth 20 times that much, or $1250/oz. (gold was almost certainly less valuable than that in most of classical antiquity, but we'll pass over that). If aromatic resins are the same price as silver, that's about five times a base price of $12.5/oz., a plausible figure in the upper half of the typical range, making it suitable for regular use by people Status 2 or greater, which seems, if not right, then at least close enough. If it costs as much as gold, that's about 100x the base price, or suitable for regular use by people Status 4 or greater. It's absolutely plausible that both of these prices could be valid at the same time, reflecting different grades of resin. The really high-grade stuff is vastly more expensive, while the low end product is within the reach of even the working class for times of special celebrations.
Oh, and a bit about usage: Like the book says, resins come in irregularly shaped "tears," about the size of a small coin, though generally significantly thicker than very flat ancient coins, so while precious metals are much denser (silver is about ten times as dense as aromatic resin), it's probably about the same number of pieces per weight, or about 5 to 10 tears per ounce. Just how much one used at any given time will be wildly variable, but casual use would likely entail 1-3 tears used at a time, chewed to freshen breath or tossed onto a brazier for a routine prayer.