Valve testers do seem to be highly prized and can be very expensive as you've found out. I never considered getting one myself but last year I finally became the owner of a valve tester when I visited my cousin in Canada who has an electronics shop over there. In the back of the shop there was a valve tester which had been left behind by a retired engineer, and my cousin said "If it's any good to you, take it!" The offer was too good to refuse!
The valve tester was an EMC model 213 made by Electronic Measurements Corporation, New York, USA. Pictures of it here: http://www.radiomuseum.org/r/electroni2_tube_tester_emc_213.html
The unit is very small, lunch-box size, just like those battery valve portable radios from the 1950s. It has a number of different valve sockets, a row of switches with neon lamps and a meter which reads emission (indicated by GOOD or REPLACE on a scale). The neon lamps indicate inter-electrode short circuits. To use the tester you must first look up the valve to be tested in the user manual, then set the switches on the tester accordingly. Testing is a two-stage process. The user manual says to carry out the short-circuit test first, and reject a shorted valve without further testing, otherwise the tester could be damaged during the next step, which is the emission test. Some valves have multiple sections, e.g. ECC83 is a double triode. In that case the emission test has to be repeated for each section.
It turns out that these basic testers were quite common in the USA and are not so expensive to buy, either. I used my EMC 213 to test a couple of new old stock valves as well as a couple of faulty valves which I had already replaced. In both cases, the tester gave a valid result, "GOOD" for the new valves and "REPLACE" for the faulty ones. I think the tester would be useful to confirm your suspicion about a valve during fault-finding. It would also be useful if you have a pile of old valves pulled from scrapped radios and televisions, and you wanted to know if the valves were likely to be serviceable or not.
The main snag with the American valve tester is that the handbook lists the valves by their American type number. For British valves I first have to find the American equivalent elsewhere, then follow the instructions for testing. Unfortunately some British valves seem to have no equivalents in the USA. If a valve is not listed in the tester's handbook, there is no way to test it. Without first applying the settings given in the manual which are unique to each valve, the meter reading is meaningless. Likewise, if you find one of those valve testers going cheap but without the manual, it will be practically useless unless you can get a copy of the operator's manual from somewhere. Luckily my tester came with an original manual, plus I found an updated copy on the Internet which allows the tester to check even more valve types.
The manual to my tester is here : http://bama.edebris.com/manuals/emc/213-215
- link at bottom of page. The manual includes a schematic diagram. It doesn't look very complex so it should be possible to build your own copy of the tester ... but you'll need a meter with the same characteristics as the original one, or the GOOD/REPLACE result will be invalid.
I'm happy with my basic valve tester (after all, it was a free gift!) though if you want something more than just a GOOD / REPLACE indicator, expect to pay more money. But before handing over hundreds of pounds for a vintage Taylor 45 or something similar, bear in mind that it's going to be at least 50 years old and may be inaccurate or faulty. There's a buyer's guide on ebay itself http://www.ebay.co.uk/gds/Choosing-a-Valve-or-Tube-tester-from-AVO-to-B-K-etc-/10000000007494378/g.html
If you have the time and patience, I reckon you will be better off building your own tester instead of buying an expensive vintage one.