I was working my way through this decision recently. I wanted a 7" tablet for the gf for Christmas as her phone wasn't cutting it as an Android device, but she didn't want to replace it and her contract isn't up for a year.
I spent ages going through all the options for the cheap tablets, and came to the conclusion that I would end up spending around £100 for one worth having, I decided it had to be Jelly Bean as most of the manufacturers don't seem to release updated firmwares very often (if they even have a web presence), ideally I needed above an 800x480 screen and I also required it to be multi-core as I have used single core android device and they are very frustrating.
I tried to pick one out for £100, went through all the usual brands people recommend (ainol, Hyundai etc.) and then notice the Nexus 7 16GB is now £159... it became a no brainer really. Proper quad core, well built device that will receive good future support (it's already been updated twice from 4.1 to 4.2) with a decent HD IPS screen and a much better battery life than most of the cheaper devices have.
Sure it's £60 more than I was going to spend, but I'd much rather give that as a gift than one I will forever have to be supporting because this thing doesn't work in the Play store, or that thing doesn't work and the only way to update it is with custom firmwares from some forum...
That said, I got mine from a well-known high-street catalogue retailer for Google's price (but not their shipping) and very few other places seem to sell it now with most just stocking the 32GB (still good vfm at £199 imo).
All of that aside, another to look out for is the Sumvision Cyclone Voyager, which has some good specs (dual core, 1gb ram/16gb rom, an actual web presence for support) but still has the 800x480 screen. That said, if you look, it can be had for less than £80 if you get into the pre-order queue.