Supply and demand. Senior designers only have so many hours to give, and the smart ones raise their rates as quickly as possible when they realize the high-budget clients are the best path to wealth and a sane schedule.

For a brand-new startup, your design is going to evolve quickly. I'd go cheap-and-quick (99Designs, TemplateMonster.com, etc). Hopefully someone on your core team is solid enough at design that you can wrangle the theme that you buy... Your design should be evolving every day.