The other possibility is there is a p-trap under there, but it is not properly vented and is being sucked dry when other fixtures (toilets) are used. I had this situation myself. The shower in the basment bathroom was not vented (just a straight shot into the 4" main). When one of the toilets towards the front of the house (1st or 2nd floor) were flushed, the water rushing past the shower connection would try to pull in air through the shower drain and take the water from the p-trap with it. So, the p-trap might be full one day and the next day it would be empty (really hear water from other fixtures, some smell, etc.).
Go to your shower and shine a flashlight down the drain and see if you see water. If you don't see any, run the shower for a couple minutes or dump a bucket of water down the drain and check again for water in the trap. If there is now water in there, you have a p-trap, but it is drying out (probably venting issue). You could also have a crack/leak in the p-trap letting the water drain out, but that shouldn't be too likely. If there is still no water, then you are probably lacking a p-trap.
Now as to the reason why it is showing up now and not before, it may be that some of the previous problems were covering up this issue. It also could be that the new work that you had done was not done correctly (did you use a real plumber and not a handyman?).
Cost is impossible to say. Even if we knew exactly what needed to be done, labor rates vary greatly across the country as well as the fact that we can't easily see access issues and other potential challenges.
Your best bet would be to educate yourself and then get a good plumber out there and get it done. I would first check for the p-trap (like I mentioned above) and then report back. Once we have those findings, you could go into the crawlspace and take some pics. The pros will be able to point out where the problem may be.