If you're looking for a number, if I really believe a domain has potential and it's currently available, I'll drop the $10 to snag the domain. If the domain is currently held by someone else and the expiration is coming up, I mark it on my calendar and check back later. If the domain is being held, has been held for a while, and doesn't look like it's coming up for air in the near term, I move on.
To answer the OP: unless you're after something ridiculous like word.com you should pay $8 - $10, domains aren't a brand until you make them one.
I also wonder why startups never seem to consider buying existing sites. Yes, your app is different from what's already there, but you could stand to gain a lot from the existing content, traffic, and incoming links that the site already has.
I was going to go with http://properprocesshq.com (which I also own) until I spoke with some clients that use our Basecamp and Campfire. They say its near impossible for them to remember where to go. They know its (someword).campfire(something).com but have difficulty remembering someword and something if they ever need to access from home... Right then and there I bit the bullet and paid up.
Chances are you'll re-brand or pivot between now and then anyway.
Successful examples: GetDropbox.com and Highrisehq.com.
Your domain is your brand.
Your brand is whatever you make it into.
Who would have thought that "Apple" would be synonymous with high-end laptop designs and innovative products?
Would "eBay" be your first thought when looking for an online auction?
Is "craigslist" the word that conjures up hookers and apartments (or now, just apartments).
And so on.
But usually, it seems that the domain matters far less than just about everything else (so long as it is usable, reasonable length, etc).