we are looking at buying a1976 pace arrow,26 foot,454 engine,it needs tranny rebiuld,has t-400 ,we have estimates for rebiuld $1100,so what should we pay for this motorhome,50,000 miles ,everything works,has generator,roof air,average condition
An RV that old probably does not have an 'official' (book) value, and also is probably subject to deterioration. Even if everything but the transmission works today, a few months or several 100 miles from now there is a fairly good chance that something else will break, or wear out. The worst would be something not as fixable as an engine or transmission, like a part which is no longer available or is inaccessable. So what you pay is a) what it is worth to you, and b) what you can afford to lose.
As a guide, NADAGUIDES.COM quotes a retail ready 1990 28 1/2 foot Pace Arrow (the closest thing to this one they still list) at between 9000 and 11000. This one is 14 years older, 2 1/2 foot shorter and not retail ready. Personally, I wouldn't saddle myself with this because I no longer have the time or energy to 'tinker', but if this did meet my needs, my thinking might be as follows:
It might be worth as much as 5000 if fixed up to 'retail ready' condition, and assuming I could get the transmission done for 1100 (including installation), I might be able to get it to that condition for (pulling a number out of the air because I don't know what it needs) 2000 (for the transmission and a few minor things, and a major detailing). This would make my target price 3000. As a sanity check, I might see if there is a book for older motorhomes which could confirm my guess of 5000.
I am not suggesting these values, they are just guesses as examples. You have to figure out what it needs to get it into a condition where it will meet your needs. Then figure out what something which meets those needs is worth to you, and if you can afford to keep in in that condition as things break or wear out. Take the value to you, subtract the cost to get it there and a fudge factor for anything which is 'just about ready to go', and that is your target price. Try to get it for less, of course.
And remember, you are not 'investing' this money, you are paying for a luxury. It is rather unlikely that you will ever get much of it back by reselling the unit.
You can buy one ready to go a little newer for $5000. You can get a 76 for around $3500-4000. Take off the 1100 and you come up with 2400-3900 depending on the condition of the rest of the coach. Remember that when you are buying something that old that you can't get parts for the older appliances. Be prepared to but new ones if you have problems.