WW - I don't know what machines you have been playing or how long you have been playing for strikes but the the Bar and Cherry machine you mention is the old original style machine, the Rooster machine at the nugget is the new style 7's machine where it takes 3 7's to win a strike, or strikes depending on what 7's you line up on a payline. I also find it hard to belive that you purchased Rooster strikes to trade for a $200 strike as this would defeat the whole program at the Nugget. I have visted the Nugget many times and they would never sell strikes.
As far as the machine you mentioned making it cost more to win strikes, you are off base here also, the odds of winning anything, credits or strikes is a set percentage which is set when the machine is built.
Walt