6$/m is too low for almost all products, you won't be able to effectively acquire customers with that. Remember, once someone gets to the point where they pull out their credit card, the difference between 6$ and 9.99 or even 19.99/m is almost negligable (although there is a difference between 9.99 and 19.99, although2 mostly you'll do better with 19.99).
Charge 19.99/month. It will give you breathing room to put Google ads, to pay referral fees (what's the word?), to experiment with temporary lower prices etc. If you charge 6$/m, you're stuck with a too low price and no breathing room whatsoever to acquire customers. Many startuppers underestimate the cost of acquiring customers - you don't just get them for free, most of the time.
ps: in most cases (unless you sell hosting or something, and even then) the price of the service has NO relation to the cost of the infrastructure and the expected number of customers. It is instead related mostly to the cost of acquiring new customers, which is likely going to be somewhere between 20 and 80$ per customer.
If it costs you, say, 40$ to acquire a paying customer, and they stay an average of 3 months, you need to charge X/month to be making money.
At the same time, there are psychological limits on what people are willing to pay. 9.99, 19.99, 39.99, 79.99 etc. Charge 8.99, for example, and you might as well charge 9.99, since you'll get exactly the same amount of customers (probably).
Seriously. Charge 19.99$/m.
Once you experiment (later) with lower prices, you'll likely see it makes no difference in conversion.
Charge 19.99$/m.