I've seen a bunch of walkthroughs saying you need at least a $1000 bounty or a $5000 bounty, to trigger this achievement. All i can say is, not true. Here's what you do, if you haven't gotten this achievement.
Head over to butter bridge, south east of macfarlanes ranch. Now butter bridge is a north/south connection, so you'll have to have Mexico open, for this to work. Now, you need to get your bounty up a few hundred dollars, north of butter bridge, then book it, heading south, over this bridge.
You of course need to be riding the hungarian half-bred for this to trigger, as well. The first time I had a bounty of $300 and the second, it was $525. It seems you just need to be heading south, running from the marshall, and this is a great place to do that.
http://freeplaystationnetwork.yolasite.com