The Keeper Trade Calculator was a tool I built along with the Draft Pick Trade Calculator in 2015.
See that write up for more on the method.
The TLDR: use past average auction values (AAV) to see the expected AAV at each pick, use that.
Once you do that, adding keepers is a natural extension, just use the player's average auction value and compare it to the value in draft picks.
Of course, keeping players makes draft picks less valuable (they're effectively later) but that's easily adjustable. Just have the calculator effectively move the picks back depending on who's kept. This meant anyone using the calculator to enter in all the league's keepers.
In my main league we just keep one player and that's it, but I know a lot of teams lose the pick in the round the keeper was picked in.
In theory, AAV makes it easy to evaluate these trades too, and you could actually hack the calculator to do decent job at it.
Same as the Draft Pick Trade Calculator version. The biggest problem is the MFL auction data is so thin. I'd really like to figure out a good way to get updated, flexible average auction values.
This was one of my first projects in React. I hosted it in a $5 droplet on Digital Ocean using Dokku. In hindsight that was completely unnecessary since everything was stored client side, but I didn't know much about web development then.