Let me try that again without terribly incompetent incoming email formatting...
I think it's a fair question.
It's one that lots of people don't think about until they're sinking like a stone having spent a pile of their own (or someone else's) money.
Best way to think about it is this.
How much do I need to spend to get each new customer and still be viable?
Google Adwords is a good example because it's simple to explain and easy to understand.
You say, my keywords are "blah, blah blah" and "tum te tum"
Average price per click for "blah blah blah" is £x, and there are y searches for it (i.e. enough)
Average price per click for "tum te tum" is £x1 and there are y searches for it (i.e. enough)
I expect a% of people who click on the link to contact me through my site (if this is the way you're doing it, if not, tell your story, above all it demonstrates you've thought about it from a customer's point of view and you understand the challenges ahead)
I expect to convert b% of those guys into a customer.
You're then looking at a cost per potential customer. As long as your logic makes sense, then you can get them to challenge your logic at each step and you'll be confident to answer it.
You can then say things like you expect a c% referral rate...etc. So for every happy customer, you expect them to attract 0.5 of another one. Or you have a referral reward system, or some sensible reason why your thing will go viral in the right community (it won't, unless you've got a great reason why it will).
You then look at other options that you're considering (bill posters, tv advertising, sponsoring meetups, whatever...etc) and use those to back up the fact you have a series of ideas as to how you're going to market the product and get people interested. Price them up in a similar way and use sensible logic to justify your cost per new customer.
You'll generally come to one of two conclusions.
a) The cost is sensible and you can absorb this and still make a profit. And you'll be able to explain this to an investor.
b) You need a new product...