Cree UR or ZR series if you're only interested in LED. Both the UR and ZR are in the same approximate price range (~$100)

http://www.cree.com/Lighting/Products/Indoor/Troffers/ZR-Series

If fluorescents are ok, you could consider using T8 fixtures - lower upfront cost than the UR series.

For T8 just buy something like these (GE 132-MVPS-N) on ebay and drop them right in if your luminaires are wired for rapid start and they are in good shape. It is really your best bang for the buck, and you can get good color choice and color rendering. You can find excellent ballasts on Ebay for less than $10 if you are patient and certainly less than $15.

If the sockets are for instant start, you have to change the sockets as well. It can be challenging to find the right, unshunted, sockets, for a decent price in my experience. If the luminaires are wired with more than one lamp/luminaire to a ballast, you need multi-lamp ballasts. You can switch to this configuration if you desire. It means running ballast cables or wires PROPERLY outside the luminaires which will take some rewiring, but cost less.

For more light, use MVPS-H and for less, use MVPS-L.