How are Chelsea managing to have a decent transfer strategy?

In recent times, we've seen Chelsea buying players and selling them on a few seasons later commanding decent fees whilst we've paid what appears to be over inflated prices and then sell our players for peanuts a few years later. Maybe Garcia and Rodwell bucked the trend for us last summer, but that was the exception to the rule. 

They paid £11M for Remy who I'm sure they could sell for the best part of £20M if they wanted to. Schurrle looks like he's going for well over £20M.
They got £30M for Lukaku and £40M+ for Luiz last summer, £38M for Mata and £20M for de Bruyne the season before.

Then they are going buying talent for what seems peanuts. Fabregas for under £30M and Costa for under £30M.

Their transfer dealings, of late, look brilliant. What are we doing wrong and who should take the blame?

I suppose the blame lies with whomever is responsible for transfers.  That and the media for telling the WHOLE FUCKING WORLD that our owner is Johnny Big Pockets.  I seem to recall Chelsea getting similarly screwed over by overpriced transfers, but none like that since Torres.

We need to be more astute and select players who are on the periphery of the verge of greatness.  But also play hardball with the cash.  Instead of going all out for Mangala to the exclusion of everyone else, for example, we should've identified three or four and play them all off against each other, see who blinks first.

Bayern were more than happy to go for Benatia (our other choice) and bagged him for less than £30M. Maybe we should have got him rather than the over inflated Mango.

Well, yes, I'd say so.  It's almost as though we're not interested in signing players that nobody else is watching, in case we get in an expensive bidding war.  Except that we then pay well over the odds anyway.

Logic doesn't seem to apply, for some reason.