My own personal experience;
You can do OK with $50 if you target correctly.
We've run $50/mo and $1500/mo campaigns (for web hosting specifically) and we pretty much got our return of investment within 60 days (client purchases 1 month, and renews the following month). Then it's a matter of keeping the churn rate low enough that your advertising dollars were spent well!
For us, we value EACH client at $1800 using the following math;
$30/month * 12 months * 5 years
Our average hosting cost for eCommerce was about $30/month and while we had an extreemly low cancellation rate, we still marked the lifespan as 5 years (many of our clients were 10+ years)
So, lets do the math;
With a $1500 spend in adwords, we needed 25 clients to signup and renew for 1 additional month. ($30 * 25 clients = $750) - so really, we needed ONE new client per day.
When you actually break out the math, Adwords is one of the cheapest forms of advertising to BUYING prospects. Keep in mind, these are already BUYERS looking for services, now you just need to be better than the competition.
Cast a wide enough net, and you'll catch fish every time!