I take it you mean by buying cheap expired web 2.0 back-links by buying expired 2.0 sites you consider high PA in order to rank a website quickly for easy search terms that don't convert all for £100?
If so, then this is an irrelevant strategy that does nothing for the site in terms of signals and will have, at some point, negative effects. Quick and easy = problems.
Then someone will have to unpick and disavow all this and it's more hassle plus wasting clients' money. Build a solid strategy and get relevant and healthy links; otherwise, it's pointless.