Is Bradley a top 25 guard in the league?
There certainly are 5+ starting SGs that are worse than him.
Guard, not just SG.
This way of approaching the is kind of pointless, by the way, given that there are better players than Bradley who are on their rookie deals, and simply players on bad contracts.
Guards who make ~$8 million right now include OJ Mayo, Jameer Nelson, Rodney Stucky, Marcus Thornton, and Mike Conley. Jennings and Dragic are in the ~$7-8 million range, and Calderon, Korver, Turner, and Kevin Martin make $6.5+ million.
Granted, a lot of this will depend on team needs and cap space, but I think that an offer in the $7-8 range is possible, and an offer north of $6.5 million is almost guaranteed. Not sure what you were expecting.
To put things in perspective, recent full non-taxpayer MLE signings include Courtney Lee, Jamal Crawford, Lou Williams, Martell Webster, and Jason Terry. Going to the $5 million level gives you Ramon Sessions or Barea.
Considering Bradley's age and skills, I think it's absolutely unreasonable to think that we can replace him seamlessly with a MLE type player. The Celtics should absolutely match any $7-odd million offer he may receive, and should probably keep him at $8 million, too.