MRS really isn't worth 4k. I just don't need it, for anything. On offense, I micro some engineers and get the same result, except they're more mobile. On defense, I build a fortress, which costs 1k less to unlock and shoots things besides. Sure it costs a ton to build, but if you need the ability to repair a large number of ships at the same time, that usually coincides with a frontier world anyway.
I don't think I'd consider unlocking MRS for more than 1.5k. I'd unlock them at that price if I'm too low on resources to build fortresses or if fortresses wouldn't provide enough benefit.