Well, I’ve just reported my second Ti-83 as stolen. I’ll probably replace it, but I’m interested in getting an RPN calculator as well. Any suggestions? (It needs to be programmable, under $30, and preferably be really old-school and rugged.)
I think your best bet is ebay. Nobody except HP makes them now (I’m happy with my 49g+, but that’s over your price range).
For the longest time, HP has been one of the main competitors to TI (in terms of calculators) and they’ve had a nice record of supporting RPN. I’m not sure what their lineup consists of for your price range, but I’d recommend checking them out. HP’s online store, Froogle, and eBay should be good places to check.
I recently bought an HP 49g+, their direct competitor to the TI-89 (MSRP is $180). There’s a good amount of information on HP’s product page for the 49g+.
Good luck. RPN is fun. 
Er, I believe phrontist was referring to an HP scientific calculator. Most engineers I know still have their old ones from 20 years ago But lucky for you, HP released a new RPN scientific calculator a couple months ago that fit your needs perfectly.
Visit: http://www.hp.com/calculators/scientific/33s/
It’s $50 (so you probably CAN find it for $30 on eBay) but will be worth it.
I just put an RPN program on my Ti89+ and it works great. Though for college I may have to get that 49G+…
I love RPN 
I have 3 of my grandfather’s really old HP RPN calculators. I think they were used for engineering or something. I have yet to figure out how to use it beyond basic math functions, but my Ti-83+ works well for that
Cory
This kid on my class made one in Java. Trig functions and everything. I’m currently making a basic 4 function non-rpn calc.