Hi Gujarwala -
I just got back from Playa and I can't confirm if the prices are cheaper on 5th Avenue but I can confirm that the price to Chichen-Itza was about $100 with going through our travel rep at the resort. We stayed at the Gran Porto Real and the tour group was Best Day. They are really nice to work with. Gran Porto Real is litterally less than a block from 5th Avenue. I don't think I've ever booked outside from the resort so I can't say what's a better deal. I will say that when we booked Chichen-itza with Go Natural Tours (through Best Day), we definitely got our monies worth. They picked us up at our hotel at about 7 in the morning and drove out to Chichen-itza which is about 2 1/2 hours from Gran Porto. The $100 included pick up/drop off transportation, lunch (which was really good), 3 hours at Chichen-itza about an 1 1/2 was with a tour guide and the rest you were on your own to look around, a stop at a Cenote (sinkhole) that you could swim in, a stop at a historic little town (I can't remember the name) but it was pretty cool. After that we were taken back to our resort. We ended up getting back to our resort at about 7, close to 8 in the evening. To me, I thought the tour was well worth the price. I don't know what all the tour comes with if you purchase off 5th Avenue but I hope my info will be a good comparison.
I hope this helps!