If you're just in need of a brand kit $1,500 sounds pretty reasonable if it's a profressional agency. Specially if that covers a name, logo, icons, colors and fonts.

I would stay away from 99Designs because if you're launching a beta you'll need someone to also help iterate any changes based on feedback. So quick and close communication is important.

if you need any further input on what kind of things you should be looking for from a designer send me an email.

I've helped over 12 startups with branding including 2 YC startups.