As has been the case before, US and CO are running various offers where you can obtain significant bonus miles if you travel on a round-trip full coach (usually Y fare).

I'd like to take advantage of such a bonus, but as crazy as it is, I don't want to pay pay a lot.

So, I'm wondering what the least expensive full coach fares might be out there? I mean if I could get a full coach fare round-trip for say under $300-$350, it might be worth trying for one of these bonuses. I would be flying out of BOS.

So far I have not found anything too appealing:
BOS-EWR full coach is about $480 round-trip;
BOS-BWI is around $500 or so round-trip;
etc.

I'm going to check PVD-BWI to see if there are any exceptionally cheap Y fares...but if anyone has seen any such Y fares, please post.

The CO offer gives you 5,000 bonus miles for each full coach round-trip. The US offer (from Marriott statement) gives you mileage bonuses for your next I think 3 full coach round-trips, and the total bonus is 20,000 miles.