The Tampa Bay Buccaneers gave Missouri Western offensive tackle Leonard Wester the biggest signing bonus among all NFL undrafted free agents this season: $20,000, per Gil Brandt and Joel Corry (h/t Jenna Laine).
Most undrafted free agents get no guaranteed money and no signing bonuses, but a few on every team get one or both. Each team has a limited bonus pool, but they can guarantee as much of an undrafted free agent's salary as they want -- though the total salary an NFL team can pay an undrafted free agent is determined by the CBA.
The fact that an undrafted free agent was paid some more money than others does not necessarily mean that he'll make the roster, or even stick beyond rookie mini-camp. Last year, the Bucs gave Ryan Delaire a $15,000 signing bonus. He didn't make the roster, or the practice squad -- and he put up two sacks against the Bucs during the regular season, having stuck on the Panthers' roster. In 2014, the Bucs gave signing bonuses to ten different undrafted free agents, none of whom made the regular season roster.
Still, it's worth knowing who the Bucs valued just a little more than other players. Aside from Wester, the Bucs gave safety Isaiah Johnson a $2,500 signing bonus, per Aaron Wilson, and safety Elijah Shumate a $13,000 bonus, per Greg Auman. The latter also reported that the Bucs gave OL Dominique Robertson a $7,500 signing bonus and guaranteed $5,000 of his base salary, and that the Bucs gave DE Channing Ward a $5,000 signing bonus.