Ibarra's is probably the cheapest and fastest sit down restaurant to eat at in McAllen. I usually go for lunch because I can get a burger, the best fries in the world, and Coke for under $5. They also serve very good chips and salsa while you are waiting for your meal. I have never eaten their mexican food so I can't comment but I had a burger the first time I ate there and have just stuck to ordering that.
It is very casual, tiny restaurant hidden off of business 83 but once you find it I guarantee you will feel like you found a treasure.