The prices are a bit more then what we have around here.. (ex. all budgies $10.00, normal grey tiels $70 or so, Lutinos $ 90..etc). The thing is the initial cost plays a very little part in the whole experience. As mysterymew mentioned, you would need to take a look at long term care and cost, time, etc. In my opinion, tiel and budgie maintanance is similar as far as diet, cleaning, vet bills, etc. My budgies personally need me alot less then my tiels do but that's because I have a whole flock and they entertain eachother flying all over the house..etc. They will both need your time and attention though. Which one do YOU like more? The better deal is for you to learn ALL you can about the one you want and do all you can to accomodate it (largest cage you can get, best food, toys, time, love..etc). Don't get one over the other because it's cheaper. It'll end up sitting in a cage by itself. Also, one thing I want to ask you is what will you be doing in 10-30 years? Going away to school, moving, anything that would effect where your bird ends up? Budgies can live 10-15 years and tiels can live 20-30 years so that plays a big role. You'll be responsible for them for their whole life and leaving them behind should not be an option.
On another note, this is just from my experience I would steer clear of Petco. Everytime I have went in there I find a critically ill bird. Their pet care (atleast here) is not up to par.