Depreciation is always a percentage of what you start out with. So if the number you start out with is smaller, then real dollars consumed is always going to be less, assuming identical rigs in identical condition.
Now having said that, you may be better off buying a more expensive used rig, compared to a cheaper new rig. In other words, buying a super low end $150K DP is going to be a piece of junk, compared to what a $150K will buy you in something that is 8 years old. Try to compare your brand new 150K rig with one of these:
http://motorhomesoftexas.com/coachrv/foretravel/2007--nimbus--36--C1692
