I found another place that is easy to farm studs when in free roam. It consists of one purple and three blue studs in Bree.
The studs are located west of the forge and they are in the building in the small room sealed by wraith bricks (likely, there was a collectible in there that needs to be collected first). With Attract Studs on, you can get the studs without going into the room.
Hop on a horse, grab the studs, and run east out of Bree. Go up the hill to the left and jump off. Go back into Bree and get near the room to get the studs. Repeat!
There is also a set of blue and purple studs on a beam between buildings to the north-northwest of the wraith bricks. You can easily get these with Attract Studs and jumping while on the horse. This set basically doubles your stud findings per run. Makes it very quick to get this achievement (you get just under 100,000,000 studs per run if you use all the multipliers).
Lil Miss Cherry was kind enough to put a video together showing this method: