It makes me wonder at times what goes through people's heads when making decisions.
The idea behind any project is to make it succeed, make it successful, the same goes for starting or running a forum - so why base any decision, especially at root level, on a 'discount' ?
Running and maintaining a successful forum is not cheap, the more popular the forum becomes the more the costs ramp up. Offer a second or third rate item and you will not get the traffic, simples as.
In business there is something thing called a USP (Unique Selling Point), basically what sets your service or product apart from everyone else which in turn makes yours more desirable.
Start with the best you can afford but bear in mind that if you come in with a low level product there are hundreds more out there the same as so why should people use or switch to yours.