Welcome but sorry you join us due to your PCa diagnosis.
As regards Proton Beam Therapy, threads have been started on this several times and it has been brought up in threads on treatment generally. (I am not surprised you were unable to find anything on it here because the search facility on this site is hopeless being packed with a lot of irrelevant information and omitting relevant information contributed by members. A recent thread reiterated this unsatisfactory state). I looked into the possibility of having Proton Beam therapy in the USA following my diagnosis in 2007 and my second opinion who was an expert on radiation including particle therapy told me that all things considered it would cost of the order of £90.000 then so probably in excess of £100,000 in present day figures. However, I believe it is now possible to have fewer fractions of a higher dose Proton Beam in a similar way that hypofractionated IMRT is gradually being rolled out following trials in the UK and elsewhere. So fewer fractions over a shorter period could significantly reduce the cost but as you say for comparable treatment in Prague it would be a lot cheaper. As to the effectiveness of it, the aforementioned expert considered it marginally better than the Photon Beam for PCa, particularly as regards reduced side effects because the vast majority of the dose is deposited on the tumour (the so called 'Bragg Peak') and little before or after, unlike Photons which a cause significant damage on the way to and after the tumour. This is of even greater importance where cancer is in the head or neck and for this reason my second opinion secured funding for children especially to be treated in the USA but for PCa not a chance! The Prague facility does not have the experience of the USA hospitals although Ashya King ( you may recall his story), has apparently done well having been treated for head cancer there that was not available in the UK. Prague is certainly worth investigating if you decide on Proton Beam treatment and you are a suitable candidate - not every man is. Insurance companies invariably pay the lowest amount they can for good treatment. Proton Beam is considerably more expensive than IMRT. Bearing this in mind, it must say something for Proton Beam therapy that American Insurers agree to pay for it extensively for PCa over less expensive Photon radiation.
If you click on my Avatar you will see from my Profile/Bio that I had IMRT with Carbon Ion Boost in Germany. (Carbon Ions pack more punch than even Protons). My treatment was within a study and Heidelberg University Hospital bore the cost of the carbon Ion element (done at GSI Darmstadt, a forerunner to the hospital's own facility) and made a reduced charge for the IMRT. I did manage to get reimbursed with the cost of the latter but the funding was given on condition I didn't divulge the source and I am legally bound not to do so.
I know Heidelberg were evaluating Proton and Carbon Ion therapy as a boost to IMRT in trials which include Prostate, although I am not sure whether these are still underway. You could contact the hospital if interested through details on their website. However, I doubt you would be treated for PCa there with just Protons or Carbon Ions, the demand on the facility is so great that they have to turn some people away I'm told. https://www.youtube.com/watch?v=LeApaY7ctMo
PS. There are other Proton Beam centres in Europe you could try, Munich has one for example.