The greatness of Moss vs. Gronk can be debated, with points made on either side. There's never been a better deep threat than Moss in his prime. Perhaps no more gifted a WR in the history of the game. But WR and TE are very different positions, and so let's take their respective best seasons and compare it to the next best player at that same position.
And as a second exercise, assume Moss is the best receiver, and thus should be the highest-paid receiver. We will take him at the highest NFL WR salary today and do the same with Gronk (relative to TE), and then with those salary restraints, build a WR/TE combo for the same dollar amount.
Ok, first exercise.
Moss (2007): 98 rec, 1493 yds, 15.2 avg, 23 td, 20 AV (by pro-football-reference.com)
Braylon Edwards: 80 rec, 1289 yds, 16.1 avg, 16 td, 13 AV
Gronk (2011): 90 rec, 1327 yds, 14.7 avg, 17 td, 14 AV
Graham: 99 rec, 1310 yds, 13.2 avg, 11 td, 13 AV
The difference between the #1 WR in 2007 (Moss) and the #2 WR in 2007 (incredibly, Edwards) is a little bit bigger than the difference between the #1 TE in 2011 (Gronk) and the #2 TE in 2011 (Graham).
Second exercise. Top WR salary = $16.2 m (average). Average WR salary = $2.3 m (average)
Moss (2007): 98 rec, 1493 yds, 15.2 avg, 23 td, 20 AV ($16.2 m)
Guys in the $2.8 m range: Beckham ($2.6), DeVante Parker ($2.7), Jacoby Jones ($2.7), LaFell ($3.0)
Top TE salary = $10.0 m (average). Average TE salary = $1.8 m (average)
Gronk (2011): 90 rec, 1327 yds, 14.7 avg, 17 td, 14 AV ($10.0 m)
Guys in the $1.8 m range: Ben Watson ($1.6), Rob Housler ($1.7), Jim Dray ($1.9), Cumberland ($1.9)
So if you had a budget of, say, $19 million, you could either have:
WR Moss, and TE Housler
or
WR Nelson (or Marshall or Antonio Brown), and TE Gronk
I think I'd rather have Brown at WR and Gronk at TE, over Moss at WR and Housler at TE, for roughly the same amount of money.
*Salary figures from: overthecap.com