Ali M's advise is right on. Given you're a start-up, therefore, an entrepreneur, I add: YOU are the most "affordable marketing expert" your start-up can hire! In addition to that realization:
- How much marketing investment (that includes the overhead) can you afford in your 5-year business plan? Add 10% and go for it.
- Do you have a top marketing strategist on your advisory board? It's likely to be the best value for money you will be able to get.