Tricky one this and I don't have THE answer. I have compiled trees for other people too and feel the same way. You say research is fun now, so number one consideration must be that it remains so. I did a tree in which I got well and truly stuck, bought the wrong certificate (who pays for that?) and that got me down to the extent I had to declare 'No more!'. This wasn't fun. It brought it home that it's a big jump to go even semi-professional.
In the past I have used the professional researchers at ROs, and they worked to a maximum of 2 hours at a time, reported their findings, and it was very much up to me to choose whether to ask for more.
However, it's also fair to say that my acquaintances at least don't expect research done to the rigorous level the text books demand. They are delighted with some charts, prints of Census Returns and a commentary. In the tree I did just before Christmas I had to say that I couldn't progress in certain lines without certificates, and that the online trail went cold c 1800 anyway, and would entail visits to ROs etc in order to perhaps get more. The family haven't asked for more (!) so I guess their curiosity has been satisfied, which is another way of saying that an introduction to a history based on online stuff easily accessed, BMD, Census Returns and similar could be offered as a package for a set amount.