I want my site to be advertised in online classifieds. Some SEO companies say they will put me in at least 50,000 places. Is this for real? How can I check to see if they did this?
"Best" and "most affordable" are a contradiction. To get the best help, you need to hire someone who is very experienced and skilled — and who therefore can command high fees. For a low budget, you'll get less experienced and less skillful efforts.
A company that says that they will "put you in 50,000 places" or "guarantee a top-10 position" is almost certainly a scam. The "50,000 places" goal is NOT SEO at all, it's spamming. The "guarantee" is usually worthless if you parse it out. In some cases, the "SEO" firm simply uses PPC bidding and thus as soon as the contract ends, your traffic ends, too.
Unfortunately, SEO requires a lot of planning and work, and takes many months to "pay off."
Using Google AdWords is also more complex than most folks understand. It's certainly a little bit easier to launch and manage, but the cost of PPC advertising is also impacted by your SEO work.