I'm not sure how others do this, but when I specify a budget...it's not so much how much I think this particular cake should cost, but how much I'm willing to spend on a cake in general. i.e. I will usually come out quoting a max budget of $250, but also say I'll be happy with anything less than this.
In both cases, my quotes have come in well under my specified max budget.

Both of mine have been 8" cakes, but I also had some sugar work in addition to white chocolate figure toppers (which adds a bit). Flavor also can factor into price and I believe some fillings (i.e. the ones with fresh fruit) have a higher price than the other options.
Just to give you an idea of prices and what we got for the $...
belle cake/ beauty and the beast cake
$150 in 2011 (red velvet w/passion fruit mousse and vanilla buttercream)
Mickey bday cake- Disney Contemporary
$137 in 2012 (chocolate w/raspberry mousse, cream cheese, and chocolate buttercream)
Both of my cakes were done by the Contemporary, but I helped a friend with an 8" cake that was done by the GF bakery too...
Allie's Sweet 16 Cake
$117 in 2011 (lemon w/vanilla buttercream)
(she supplied the topper portion from home)
So, not sure what your max budget is for a cake, but this should give you at least some idea what people have paid for 8" cakes. So, a lot will come down to how much decoration you want and flavors you want.