If you're not building a physical product, I don't think it's necessary to invest in a domain. Early on you should just be focusing on the product. The name may change as the product evolves. If your product reaches any level of success, you can then decide to purchase a better domain. At the point of success, you'll have the resources to purchase a better domain.
Keep in mind...
* dropbox.com was getdropbox.com
* basecamp.com was basecamphq.com
* foursquare.com was playfoursquare.com
* facebook.com was thefacebook.com
There are a lot of creative ways to fake it until you make it. Don't choose a terrible name that nobody can pronounce, but there are other alternatives for a lot less money. In the last year, I've purchased a half dozen domains that are simple to pronounce and didn't cost more than $9.