The valuation will depend upon what it requires to make that $5. This might be time, CPU cycles, RAM, bandwidth, or any other requirement.
The valuation also depends upon how stable the revenue is. Proxies, for example, tend to have low valuations, because their revenue is often short-term.
Valuations can range from as low as three months revenue to well over sixty months revenue.
Smaller sites tend to get lower multiplier. At $5/mo, the site is probably worth around $40.