Hi;
please, do not take it as simply the least powerfull. From some time ago (take more than a year) we've been discussing the "collectible X actual value" battle. Many long threads were issued to this subject.
The low end models like the HP21, HP31, HP10C (big target) are rare for many reasons, being thrown-away one of them. They were produced in low numbers and for a short period of time when compared to the top-level ones. You may simply compare it to the other Voyagers, HP12C not counted, and see that the other three are relatively easy to find at e-bay. No mention to theyr prices...
The HP10C without the box (a separate issue, indeed) have already reached and being over US$400.00. A complete, mint unit may be sold for about US$600.00 or more, depending on the buyer. That's what collectors ONLY are for (many contributors in here are collctors and users, I'm not talking about them): raising collectible items' prices.
Being a powerfull calculator will not change the fact it is a rare calculator or not for collectors.
I use and work with all of my calculators. Well, some are used a few times, like the HP25 and the HP31. One that is teasing me is the HP55. The only one that actually perform x<=y and x=y by the keyboard. Yes, it jumps to the specified program step if test is true or keeps program in current step if test is false even if you perform the test by the keyboard. AFAIK, no other model allows it.
Hope this helps understanding why is the HP10C so expensive. It's a fictitious price, not an actual price.
Cheers.