The Cuban peso is available at between 20-25 to the Convertible Peso (CUC) from the Cadecas but to all intents and purposes is worthless for tourists. The hotels only deal in Convertible Pesos (CUCs), the only shops which have something worth buying conduct all their transactions in CUCs, all the local bars and music venues will expect tourists to pay in CUCs and will refuse you admission if you try to argue otherwise and the locals know how worthless it is to you and won't thank you for tipping them in local pesos unless you give them at least 25. And the Cadecas will refuse to change them back for you if you have any left over at the end of your stay.
You just have to accept that there is a dual economy in Cuba and clause No1 in the job description for 'foreign visitor in Cuba' is that you will pay far more than the locals do for anything and everything. If you have a problem with this you'll have a problem with being a tourist in Cuba. Given that even the cheapest of packages to Cuba will have cost more than what most locals earn in a year (even most docters only earn the equivalent of about 100CUCs per month) then I think that the Cubans are justified in assuming that you can afford to pay a lot more than locals do. If we knew that the vast majority of visitors to Britain where paying anything remotely approaching the average annual wage in the UK in order to spend a fortnight here then no doubt we'd do the same.
That said I do usually get just a few pounds worth of local pesos because they are handy for buying fruit from street sellers, it's worth going into the non-CUC shops just to see why the locals are so desperate to get hold of CUCs and not have to shop in them and if you want to risk life and limb on taking a ride on the converted 3 ton army trucks that are used as local buses then you need local pesos for your fare. But you're not likely to have any other use for them. Any I have left - and I always do even though I change less on each visit - I simply give away to someone local, making it clear that it isn't a tip just a way of me getting rid of something useless to me that does have some value to them. Anybody that I am going to tip I always do so in CUCs or in kind or on most occasions both.
SM