Hi,
today I have been here:
hotel-tainan.com.tw
It is very close to train station, and has two restaurants. One is dedicated to outside (people not staying in hotel) and is an all-you-can-eat, on the left after you get in. If you go from noon to 2PM it costs about 750NTD during the week (not sure about the exact price beacuse I was guest and did not pay…), weekend is more expensive, still reasonalbe. It also has an happy hour after 2:30PM; I have been there during that afternoon low-cost rate but the food is cheaper (more similar to night market things).
I have to say that it is the best all-you-can-eat I have ever been in. It has a very large choice from sushi to all kinds of fish, hot pot, fruit, etc. When you get there (I think you have to get a reservation) they also park your car: just go in front of the entrance and give them the keys. Two years ago there was more choice in the fruit, but still a good restaurant 
Don’t expect western food though.
That doesn’t sound like a very good price unless liquor is included. Any other all-you-can-eat places?