Here is what I do every year. This doesn't work for everyone, but this has been my way of doing things for the last 3 models of 13" MBP's.
I get the latest low end with some kind of discount (MicroCenter, edu, EPP), with tax ends up around $1175. When the ~ 1 year mark rolls around, the refresh is usually out. I again, get the lower end 13", which usually outclasses the previous model higher end 13" (example: This years i5 is much better than last years 2.66 C2D)
I sell my MBP online for usually between $800 and $1000. On average, a $350 loss.
The cost, with tax, between the lower end 13" and the higher end, is around $330.
So every year, I get the newest 13". And all said, it costs me about $300 - $350 to upgrade each year. Each 13" starts with a brand new warranty, etc.. So I pay $350 to have the latest, low end, 13". This model works well for me. It may not for everyone.
I rotate my SSD / 8GB of ram into each new model. This year, I had to upgrade the memory to 1333 (so an additional $65) - but was able to sell my 8GB 1066 for $50, so really on $15.
So really, it's kinda like a lease.
I know that each year, I'll pay about $350 to have a warrantied, latest gen 13". I also get the other little next gen nice to haves. So for instance, this year, I have a larger HDD (even though it's since removed and using as backup), 6Gbps Sata, Thunderbolt, better wifi, etc... From the 2009 --> 2010 upgrade I did, I got longer battery, increased HDD, increased CPU, better GPU, etc...
I avoid the temptation to make the upgrade to the "high end" 13" - because the cost ($350) would mean I'd have to go 2-years between cycle. And experience shows there is usually an 8 - 13% difference in computing power between the models, for 28% price premium. So numbers wise, low end is for me.
Next year, when the new ones are out (with likely a next gen i7 in low end) that's what I'll have. And it will beat this years i7, but not be as good as the higher frequency i7 that will go in to higher end 2012 MBP.
