First decide how much budget you want to put into Marketing overall. Then decide which Marketing programs will be most important to the startup's early growth. The usual suspects include SEM/PPC, email marketing, PR, event marketing, social media, and (yes) SEO.

If your startup is early stage, then allocate as much budget as you can to the 1 (or 2 max) most important initiatives. You're probably too small to do more than just 1 (or maybe 2) well. If you do lots of programs poorly, you'll get no results at all. It's better to do just 1 very well than 10 poorly.

If you're so early stage that you have no idea which programs to invest in, let's talk. I can help you figure that out based on your business model.

If you're a later-stage startup and you already know which programs work and which don't, then you'll inevitably start running into the problem of scale. For example, your CPC for PPC ads will rise as the quantity of clicks you get rises, until it's not worth increasing your PPC budget. SEO is a great way to scale beyond what PPC can do for you. So when you git decreasing marginal returns on PPC campaigns, start putting the budget you would otherwise put into expanding the PPC campaign into SEO instead.