If you're going to pay for advertising, pay in such a way that the people who click your ads are likely to give you a link.
Don't try to advertise in such a way that they're going to click an ad on your site so that you make money. Even if you can break even this way - which is unlikely - once they've clicked someone else's ad, you've likely lost them forever.
Even if you make money directly, as with eCommerce, try out this calculation: which is more valuable to you? The money you make when a visitor buys one of your widgets, or when they give you a link?
By comparing how much money I've earned from my site over the years, then checking Google to see how many inbound links I have, I can easily tell that depending on the situation, a SINGLE link has already been worth from ONE HUNDRED to ONE THOUSAND dollars to me.
I run AdSense, so if they click one of those ads, I make a few pennies.
It's easy for me to decide - when I advertise, I advertise those of my pages which do not yet have many inbound links, but that I think are appealing enough that visitors will enthusiastically link them. I don't even try to use advertising to drive traffic to my ads.
Also important is to develop deep links. Don't just try to get links to your homepage. That's make your homepage do well in the SERPs, but what if someone's not searching for a keyword that's mentioned anywhere on your homepage? Instead, try to build links to each and every page that has content that's likely to be valuable to a visitor.
It's easier to build traffic to a page that presently has low PR. For keywords that aren't competitive, you can often be in the top ten or even sometimes #1 with a PR5 page.
Paying for advertising to take a page from 5 to 6 can be expensive, but not so expensive to take ten separate pages from 4 to 5. If the pages are all about different keywords, you can draw in traffic from a wide variety of searches, and then if you have good navigation, you can get the visitor to explore the rest of your site, and then either click the ads you publish or buy your widgets.