Hopefully you've already taken the MCAT in which case that cost is already paid for.
Apply to around 15 schools, which the average. It's 30 bucks per application on AMCAS. $450
Then, let's be generous and say you get a dozen secondaries. They're almost around $100 a pop. $1200
Now let's say you get 6 or so interviews. If you're flying, plane tickets and cab-fare (never forget about cab-fare, it's a huge cost), it can be around almost 350 or above for each one (low estimate, but we'll assume you're driving to some). $2100
So, you can spend upwards of $3,750 on the application process (depending of course on how many interviews you get), and assuming you have a suit already. One thing to do is schedule interviews smartly. I know lots of people like to build up to their top choice so they schedule them later. I scheduled it earlier, heard back, and then could safely cancel some other interviews saving me airfare to Nebraska and Oregon. Just remember, this whole thing is an investment. That combined with the fact that 4K in debt is a drop in the bucket of the 200K we'll be in at the end of this thing.