Following may be of use...http://www.arunet.co.uk/tkboyd/ec/ec1ems.htm
Cheap? Not very ($40)... but if you have a lock on the door, presumably you have something expensive behind it?
As for "no fail-safe way to get out"... you can fit the strikeplate with an ordinary lock, and however you would get out past that lock is unaffected by the strikeplate... one of the advantages of taking that route.
Note that you need to be talking about a "tongue" type "bolt", which pushes back for closing/ locking the door. ("Yale lock".)A rectangular type "goes into a mortice" type bolt won't work.