Your website is your most important asset as it is the window to the world these days. Of course, if you are driving traffic from Google through organic search, you have to be very careful as to the kinds of tactics deployed.
See what happens when a “cheap and supposedly good” SEO agency uses risky moves on this website. Traffic fell off the cliff and business just tanked when it was penalized.
The company was lucky to be able to get out of the penalty when they employed a new agency (guess who? *wink* 
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Some are not as fortunate and they may need to totally trash the domain and start everything from a new one.
Moral of the story: Cheap is not necessarily good and good is not necessarily cheap. Be careful!