You will get at least 30 000 credits for 100 crystals, but often you are awarded with more. Sometimes much more. If you plan to exchange a lot of crystals for credits, and you get a big bonus, you can double it (Say that you got 42 000 credits. You can now select to spend another 100 crystals to get 42 000 credits more.)
Until recently it made little sense to exchange crystals for credits. You could farm credits easily, as bropoints were very generously awarded until a recent update. For those bropoints you could buy credits and your fortune of credits increased fast.
Now, bropoints are much more difficult to work for, so the balance between crystals and credits has changed. Today (June 2017), it seems to make more sense to evolve crystals to credits. But you should always use caution, when trading crystals to credits – you cannot let the affair go back and there are just a few ways in the game to earn more crystals.