Felton's tender is at the original-round level, so he is worth a fifth-round pick if he signs elsewhere and the offer isn't matched. The Lions drafted him in that round back in 2008, and he has been the team's starting fullback for the most part ever since.
Earlier in the offseason Felton said that he wants to be used more or play somewhere else. Because he has only been in the league for three years, he is a restricted free agent now and probably will be in the new CBA as well. That means the only way he can play elsewhere is by getting a team to sign him and give up a fifth-round pick. Of course, the Lions would have the right to match any offer made to him, so in the end it wouldn't be up to Felton anyway.
It is unknown what level the tenders are for Rayner and Fluellen. If they're also original-round tenders, Rayner was a sixth-round pick in 2005 and Fluellen was picked in the third round in 2008.