I'm curious at the idea that you shouldn't expect a return on a £75 investment.
As a results-based marketer, I expect a return on every penny I invest. Granted, sometimes that return is in learning that a strategy or system doesn't work in a particular scenario; but with £75, you could easily set up a campaign, create a landing page, and (depending on competition and how much effort you're prepared to put into learning the basics) start delivering traffic to your site.
My advice? Start with token daily budget - maybe £2-3. The beauty of the "variable cost" model is that the results are, largely, scaleable. So, if you start to see small results with a small budget, the likelyhood is that you'll get better results with a bigger budget.
But - and it's a big but - lay your groundwork first. Get Analytics and Conversion Tracking set up first... otherwise you won't have all the facts when you decide whether to put real cash into it!