It should be:
THE CHEAPEST FITNESS CENTER & GYM IN TOWN
Once we have changed the word CHEAP to CHEAPEST, we do not have to add the word MOST anymore. It is also wrong to say MORE CHEAPER, as the same rule applies here.
This is a common error frequently made by Malaysians when it comes to the usage of comparative and superlative adjectives. This happens most probably from the direct translation of Malay language for phrases like "LEBIH MURAH" and "PALING MURAH".