If you are strapped for cash or in the mood for a bit of exercise, one can walk up to Monte Alban from the city of Oaxaca. I recommend, however, taking the local bus for 5.5 Pesos, which stops about 2/3 of the way up the hill to the ruins. Just get out at the bus stand (a dirt parking lot, which is the final stop for the buses before they head back down the same hill again), and start walking up the hill. It's a good 30 minutes to an hour assent, depending on your fitness level and the heat of the day (I suggest an early start). It's a gradual climb, not difficult, but there isn't much shade, so be prepared. A 57 Peso (current as of Aug 2012) entrance fee gets you access to the ruins and the small but worthwhile on-site museum, and if you arrive early enough, you'll beat the crowds and can even watch the vendors too busy setting up their stalls than to try to sell you their wares.
A 20-30 minute downhill walk to the dirt bus stand, 5.5 Pesos to bus it back down to the city and you saved yourself a heap of change.
The site is lovely and if you are fortunate enough to go on a clear day you'll have amazing views of Oaxaca and the valleys on the other side of the ridge. Enjoy!
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.