Based on my (limited) experience:
Some watchmakers will refuse to work on reps...some that work for a franchised jeweler may be prohibited from working on reps...I doubt the local Rolex dealer allows their watchmakers to do service on rep watches, for example. Many independent, "mom and pop" watchmakers will pretty much work on anything you bring them, although with some reps, especially ones with proprietary, cloned movements, parts sourcing can be a problem. A Submariner clone with an Asian 2824 movement can pretty much be serviced by anyone. An AP or Omega with a semi-cloned movement might be very hard to service and parts can be impossible (or nearly so) to find. Overall, reps are known to frequently come with dirty movements and insufficient lubrication, due to their being manufactured in less than sanitary factories...some reps have been reported with movements lacking any lubrication at all. Some have hair, dirt, skin cells, etc in the movement. A service can do wonders for the reliability and longevity of a rep, but it may not be cost-effective, depending on the watch.
A service on a rep will generally cost the same as the same service on a genuine watch. It depends on how complicated the movement is (manual wind, 3-hand movements with no date...like a Rolex Explorer...are cheaper to service than automatic chronographs with a moon phase, for example), the extent of the service (simple cleaning and lubricating is cheaper than a full teardown, parts replacement, and case polishing, for example). So anywhere from $100-200 up to just shy of $1000. It's really hard to say. It's like quoting how much to rebuild the engine on a car...it depends on if you have it done at a dealership with all new, factory parts, or by the local shop with rebuilt and aftermarket parts, or by the shadetree mechanic who uses whatever he can get cheaply.
Service and waterproofing is kind of a personal, case by case thing. On many rep watches, completely replacing the movement with a new one is cheaper than a service. Sometimes, buying a completely new watch is cheaper than a service. It all depends on how much you like the watch, how much you want it to last, and how hard it would be to replace it if it breaks and can't be fixed. Using cars as an example again, most people would pay for the dealership to stop an oil leak on a Ferrari...most people would just throw an extra quart of oil into a 20 year old Chevy every month. Most reps will tolerate day to day hand washing and splashes, even swimming in a pool. Since the design closely replicates a proven design, most reps approximate the water resistance of the gens, assuming proper assembly (ie, all the gaskets are installed properly, which is not always the case) and the crystal, crown, and case back are properly secured (which, again, is not always the case).
Generally speaking, a watch...even the cheapest rep...is a precision mechanical device; one that will last nearly forever if properly cared for and maintained. How much you invest into keeping it running, however, is a personal choice, depending on a lot of factors.
Ultimately, many here have learned to do some basic servicing ourselves, since there's not a huge barrier to entry (the cost of one service can pay for enough tools and materials to start doing it yourself, and You Tube and forums like this have an abundance of knowledge to help you along). A movement serviced at home, if done correctly, is a cheap way to get the reliability without investing an extra $300-500 into every rep that comes along, and much better than leaving them to self-destruct because some dust is slowly grinding the gears into mush.
