Maybe this is a pet peeve of mine, but I really hate the phrase "give away" in the context of equity. It's like nails on chalkboard, at least for me.
You are selling equity, not donating it! What price should you charge? Whatever the market will bear!
If you can't elicit an equivalent second offer (i.e. another investor seeking similar terms who brings similar expertise to your business), then you should either accept it or - if this is your first priced round - delay pricing by trying to negotiate for a note, as Steve suggests.
If you can get a second offer, don't auction the shares back & forth, but pick an aggressive number you see in the market for your stage and ask either if they'll agree to it in order to close. If neither bites, listen to both and pick a slightly lower number that you think won't upset them until one says yes.
Either way, valuation is the least of your problems right now. Here's what you should be worried about:
1) Is this enough cash to get you to safely a next significant milestone? It's probably a much better idea to try to raise more money than $200k when selling 15% of your company than to sell, say 10% for $200k instead of 15% for $200k. At your stage, if you don't like your valuation, deal with it by asking for more cash, not by trying to sell less for the same amount.
2) Liquidation preferences.
3) Anti-dilution clauses.
4) How do other investors view you after you close this deal (more likely to invest next time, less likely, or indifferent)?
5) Any accumulated interest rate hidden in those terms somewhere?
Your valuation is not your valuation... It's your valuation under a very specific set of circumstances. Think about all possibilities before you sweat what happens in the best case.
Also, the terms of your first priced round set precedent for later rounds - valuation does not. So the terms of this document will keep coming back to haunt you no matter what. The valuation only impacts you if you're wildly successful. Focus on the terms or delay these decisions by using a note.