My guess is that each of the retired builder would have different prices in mind.

Just make sure you make it very clear as to what you expect and write it all down and have both of you sign it. For example, you expect "XXX" to be completed, you want this "XXX" modification/addition, you would like to be able to check on the build every 2 weeks or so, the anticipated date of completion is... etc...

Not that you don't trust each other, but just so that there is no confusion as to what was agreed upon. Back in my 'youngun' days a handshake was all that was needed but now it's best to spell everything out in writing,