Hmmm, I know you said you don't want to spend a lot, but I have to still recommend a graphing calculator for the fun and the mileage! There is soooooooo much to learn on those, and mathy kids in particular really enjoy them. The "cheap" but good one is the TI-83, that will still set you back a good $60 or more.
You really don't need that for the Explore though, and literally any basic "scientific" calculator is worth the money these days. You can get some pretty generic ones now for about $20 (guess).
Once the kid is doing algebra though, the graphing calculator is a huge enhancement. I think our schools start playing around with them in 7th (non-algebra) grade.
FWIW, DS7 doesn't have a graphic calculator yet. He has a cheap (8 pounds IIRR) scientific calculator which is all he's going to need in a test situation for some time, and for graphics exploration he uses
Geogebra - which is free, and which I highly recommend. Maybe graphical calculators do a lot of other stuff I don't know about (I've never owned one!) but the stuff I hear people talk about can all be done in Geogebra, and surely it's better to have it on a full size screen... So there's another option to consider.
[ETA: Ah. Very old thread, resurrected by spammer. Sorry! Regardless, I still recommend Geogebra :-) ]
