For low dollars and the bonus of potential portability, could get/make a CHA47 or similar variant. This amp works remarkably well for Grados. If you're handy with a soldering iron, can put one together yourself in a matter of a few hours/days (depends on your facility and parts you select) for < $40 or $50 (again, depending on parts) or get one of the DIYers here to make one for you for a couple bucks more. And, you can customize it the way you want... can make it non-portable or portable, can put it into a cool looking case, a mint tin, or a more standard looking case. Totally up to you!
To my ears, this is a tremendous bang-for-the-buck amp that pairs quite nicely with Grados. No, it's not an uber-amp. But it's a great way to get introduced to some very, very good sound.
If you wanted to add a little toob goodness to your rig, would second the rec for the MAD Ear+. Very solid and capable amp that matches nicely with Grado. Can also drive higher impedence cans (e.g., Senn HD600) as well. Can also change tubes to suit your tastes. Very versatile option.
Again, if you're handy with a soldering iron, can get the kit and do it yourself. Will save a bit of money (if you already have the tools) and will have the joy of building your own gear. Even if you haven't ever touched an iron, can get the kit to build yourself. It's going to ultimately cost you more... you'll need to buy some tools to build the kit. But the kit is VERY straightforward to build. Heck, even I was successfull getting it going on the first try, and I'd never done any electronics work before. And, durn it, the build was fun!!!
If you don't want to mess around actually building it, can get the fully built and configured Ear+ Purist amp for < $400 (now with upgraded Black Gate caps and Alps stepped pot!). See the MAD
website for more info. Also, search this site to see Nick Dangerous' review and other comments about the pairing of MAD + Grado.
Bruce