Yes you can pillar a syth. stock. Advantages are compression - in cheaper plastic or wood stocks that compress when tightening the action screws( for the average shooter, anyway). By far glass bedding is seen as the accuracy gaining procedure of the 2 but many just get them both done in wood or synth stock at the same time.
In wood stock they go hand and hand IMHO. However in a plastic syth or tupperware (aka most factory syth. stocks) the glass bedding is the big pay off and often the pillars aren't really required. They never hurt though. Personally on factory plastic stocked rifles I glass bed/free float barrel first then if I feel necessary I will add pillars later.
I never do either before getting a baseline, unless building a custom rifle is the goal. This way you know what it did previous and what the changes amounted to in actual gain.
100 bucks should get you the glass bed (unless you feel comfortable doing it on your own - not real tough but some don't want the bother. If at all leary take it to a good gunsmith.) 100 bucks or so should get you pillars, however it depends on the block used/preferred and process required for your rifle. This also can be done at home but requires more then a bedding kit. For most best left to the professional