A reasonably priced, but good tail light is the Niterider Solas. It is Li-ion (and this needs to be charged through its micro-usb plug), is 2 watts (it's really strong), and it has excellent side visibility too, which arguably is better than using trail headlight with a red lens (which will then have no sidelight whatsoever).
https://www.google.dk/search?q=Niter...cwsPfhBNLNgPAG
I bought mine from Amazon.de, but I believe they are American, so it should be a little cheaper over there.
I view the tail light as possibly the most important light, as with the headlight I can take action, whereas with the rear, I need other people to see me and take action if need be. Luckily, you don't need a "to-see-light", but you do need a be-seen-light - and one that also throws light to the sides to be the safest.