When I started my first company (long, long ago in a valley not so far away), I learned a valuable lesson from Jim Fitzsimmons, the guy I recruited to be CEO.
Jim had experience both as an entrepreneur, and as a corporate manager (he had been assistant controller of all of Pepsico), and he had a favorite saying:
"Chris, you get what you inspect, not what you expect."
Translation? Unless you can define success in measurable terms, you're not likely to achieve it.
This is why metrics are the lifeblood of marketing. Just as professional military commanders understand that logistics are usually more important than the oh-so-sexy field of strategy, so professional marketers understand that metrics are more important than flashier cousins like branding and positioning.
Unlike Sales or Engineering, where everyone knows how to measure the results ("How much did you sell?" "Does the product work?"), Marketing is not blessed with such simple metrics. While it is important to be in the right sector of Gartner's Magic Quadrant, inclusion or exclusion doesn't determine the fate of your company. And don't forget, Pets.com had great brand awareness.
There are marketers that slide by on pleasant talk and cool advertisements--these are often enough, especially in large organizations, to satisfy questions about the marketing budget--but startups don't have that luxury.
It's a challenge, but you have to pick the metrics that matter, and then manage to them.
My personal philosophy is to manage sales support and branding/awareness separately. Measure your lead generation programs on the sales they generate (and how cost-effectively they do so), and separately agree on the amount you're willing to spend to generate awareness and PR.
Even then, I believe it is important to set PR targets such as # of mentions and article placements. Otherwise, it is far to easy to spend $20K/month on a PR agency without knowing what you're really getting.
Figuring out what to measure isn't easy, but if you do a good job of defining your goals, it makes your ability to judge your success (and justify your budget) far greater than relying on that old-fashioned blarney.