I'd suggest $9.99 with a "trial". Give them a few calculations for free and then shut it down until they register. I'd also suggest setting up a free web vers

I'd suggest $9.99 with a "trial". Give them a few calculations for free and then shut it down until they register. I'd also suggest setting up a free web version but under a completely different name and domain. Slap on some relevant ads (Adsense, ebay, etc). You're note going to destroy your market either way. You're not making anything right now, so experiment!