I did get 99 thieving as you asked.
This is how I did it: 1-20 Cake stall, 20-50 Silk stall, 50-60 Silver stall, 60-80 Cake stall (got tired of the silver stall) 80-99 Gem stall.
I got 4,51M from that. I'll edit in information about those two "ways" I told you about ingame.
Yes... I'll test both the ways to get the info you need.
Edit: I finally finished the slow and profitable way. I did like last time, but this time I kept on doing silver stall until lvl 80 and then I went on to Gem stall and got 99.
This time I got the exact amout of cash I got by doing this. 9.629.150 gp.