Your comments and experience are very appreciated. As another user pointed this out too, we'll prepare a small example list to be easier to understand pricing.
We are practically focusing on quality but we are doing our best to keep costs low for everyone.
Either way, clearly you're already calculating this value for billing, no? Or are you just using laser run-time for the bill, and estimating that based on the user's guesstimated path length?
With vectors imported into a common data structure you just need to convert curves to line segments with enough segments to maintain a reasonable approximation and then add up their lengths. Cairo and Anti-grain geometry can do the heavy lifting for you there. Anti-grain has example code for doing this adaptively.