Just sourcing the commission Bay makes on the Movie toys. (I knew I heard it from somewhere)

His deal with Hasbro is second only to that of George Lucas, who gets an estimated 15% royalty on all Star Wars figures. Bay gets an estimated 8% on Transformer toys tied to movies.
The entire article is an interesting read. Explaining how he earns so much now, due to being paid small up-front fees in his early movies, making other people rich off his work... so now he negotiates a post-revenue payment, to make a percentage of all profits of all items sold off his "product" including DVDs and toys.
It has made him one of the highest paid people in Hollywood in recent years, with about US$125 million in 2009 - breaking down to 75 from Movie profit, 28 from DVD sales commission, and 12.5 just from the TF2 toys commission.

And that was for the second movie, which he wanted to do... one can only imagine what deal he got for the third and fourth one, to get him coming back.

His earnings for TF3 was said to be about US$160 million, on top of the 125 for TF2, and 82 million last year... I don't know what he is spending his money on, but with the money he is making each year recently, he was apparently only worth US$200 million after TF3.

TF1 earned him 75...
TF2 earned him 125...
TF3 earned him 160...
TF4.... wow, he could be making over 200 million this year.