It sounds like a pretty good strategy, if you stick to it.
I have been doing something similar, with the exception of buying into mtt's that cost less than 5% of my BR. These mtt's are on a South African based poker room, and they are pretty small. I cash in about 20%-30% of the mtt's I play in and my BR is growing slowly. If I make it into the money, I usually finish to 4 or 5, winning 1 or 2 a week.
My only problem is that every now and then, my patience gets the better of me, and I play in a ring game and lose a chunk of my BR, or play a much more expensive mtt, then have to rebuild.
Patience is key here, and I have to keep myself strictly to the strategy!