[Batman]

Just a thought from a plumber....As explained in broken typing/english by Garrett.
1st test the base. Cover the outlet of the shower with a plunger/cloth/glad wrap and fill the base gently with a bucket being careful to only get water on the base tiles, keeps filling to just below spill level where the screen meets the tiles, make yourself a coffer & go downstairs and wait and watch. If you get no leaks after about 15 mins then let the water out & go back down to watch again. This second time will check the integrity of the waste pipe.
2nd..Check the pipework in the wall. Remove the shower rose, cap it off, take off the handles & flanges and turn on the cold tap only. Leave under pressure for an hour and then check around spindles and also wall behind shower.
3rd...Check the grouting including corners, the seal along shower screen, check the shower screen closes properly and that the rose doesn't shoot water at any gaps in the screen. Also check the grout seal where the floor tiles hit the shower base.

If you have no joy with any of these options then you may want to cut a small hole in the plaster from downstairs where the stains are and look in with a torch & mirror, with the shower running.

I have found (after 15yrs doing this sort of thing) that this process of elimination can sometimes save you from big dollar spending straight away. Little by little is the go.

I have heard of the companies that re-seal showers & the price is about what you've been quoted but look for others that will do a fully comprehensive job & guarantee the work they do. Some mobs down here back up the work with a 10yr guarantee.

I know it seems like a pain in the ass but you're making a major investment in the house so you want to make sure you get it right. If all else fails you should be able to rip out the whole bathroom and remodel it for under $10k.. EASY The most expensive renovation I ever did for a paying customer was $9,200 & that was an upstairs BR with bath, shower, basin & wc including demo, plumbing and tiling. Remeber that you can always do the demo yourself and load bins & carry boxes of tiles upstairs etc. Anything you can do to keep the cost down would help.

I must go & have another drink, all this typing is making me thirsty.

Cheers
Batman