Keenum is a free agent after this season, so no compensation (except possibly a comp pick which depends on any free agents the Rams sign) for him. Foles makes $12 million next season, which would reduce compensation for him significantly, especially since he's a free agent after that.
The chance of a mid round pick for Keenum in a year has to be factored in for him, of course, when the Rams decide what would be an acceptable offer for him. A future 7th would be laughed at. A first this year would likely be accepted. Much more likely that any offer would fall in between, and the Rams would have to decide if it were acceptable. I don't expect Keenum to be traded. I think Foles likely will for a late round pick or two, or as part of a trade to move up. That $1.75 million salary for 2016 has to be tempting, after all.