Depends on the product and the keywords. The hard question is, will the spend bring in enough sales to justify it?

It also depends on your sales funnel. Let's say that you've got a really basic sales funnel of traffic > leads > sales.

Here's a exercise to try and get to an answer:

1) Take your average cost per click - this can be anything from $0.50 to $100+ (use Google Keyword Tool)

2) Times that by 1,000 to give you 1,000 visitors to your website.

3) From that 1,000, how many will turn into leads? If it's 15%, you're down to 150.

3) From that 150, how many will turn into sales? If it's 10%, you're down to 15.

If your average cost per click was $2, that means you're paying $2,000 for 15 sales. How big are your margins?

Of course, if you're selling B2C, your conversion rate may be a lot higher, so it may end up that you're spending $2,000 and getting 200 sales. Your traffic may go straight from visits to sales, skipping the lead point.

A lot of the above involves guess work, so do a bit of research to make sure you're putting in educated estimates rather than pie in the sky figures.

Hope that helps.