The RT3 is a neat organ. What really sets it apart are the AGO pedals. On the right the tabs are additional settings for the pedals, although on most of the ones I've seen they don't work. But that's ok because the pedals are perfectly usable without them. Other than that, it truly is a B3 in a church cabinet. They don't bring as much, though. I think it has to do with the weight. They are considerably heavier.
For one in working condition, I've had a dealer tell me he would pay no more than $800 for one. There are a couple on eBay with a buy it now for around $1500-$1800, been there a while with no takers. Price also varies according to location too. I have seen some deals on Hammonds on eBay, only to find they are in your neck of the woods and the additional shipping takes the edge off the good deal. The price will also vary depending if it is an organ alone, includes a tone cabinet, or a Leslie.
It sounds like your client's organ has sat without oil for some time and the tone generator has either stuck or it is tight. It's possible to get it going without too much fuss, although sometimes they require a more serious effort.
To start with, you need some Hammond oil. Enough for more than one oiling. The purists might well turn their noses up at this, but I always try to keep the costs down for my clients so I am willing to go outside the box when it will work.
Then oil the organ. I will fill the two cups on the right two or three times to get the oil flowing. On the motor, only use enough oil to saturate the felt in the little bathtub. Too much will make a mess of the vibrato scanner. Let the oil soak in for maybe an hour. Then try to turn the generator by hand. You are looking for it to turn very freely. If it does, try starting it. If it runs you are home free. If not, let it sit a couple hours and try again. The oil has to get to the bearings and this could take hours, days, or weeks. If it is still tight, wait until morning and oil it again. then repeat the procedure. If the generator feels free but it won't run, it could be the run motor - this would be where the start motor brings it up to speed and the run motor does not catch. If this is the case, check the run motor - the one on the left of the generator. It should spin freely, but ALSO have side to side play in the shaft. This is essential so it is able to find a "sweet spot" in the magnetic field. I'd estimate at least a quarter inch side to side play. If it does not have it, just work it side to side until it loosens up.
That was the easy way. Sometimes they are stubborn and you have to be persuasive. This involves pulling the manuals and removing the generator. Then you oil each individual bearing by hand to free it up. Lots of wires to un-solder and put back. That's why I will try it without removing the generator unless the client needs it right now. It's the difference between oil and patience, and maybe $400-$500!