RW:
I have no idea about the legal binding of accepting awards on SIMS/SURFS. My guess is that, if you withdrew, you would not be bound to anything, since they would likely just "delete" your account. The money destined for your package would then be re-distributed to other students at UMB. I think you have a few weeks before you make a decision, however. OFA sends me periodic nagging (in a good way) emails if I forget to submit or fill something in - my guess is that they'd do the same for you. I think that this is a question better directed at OFA, though. When do you have to decide by? May 15? I remember that day 2 years ago...bleh, let's not go there.
To give you an idea of my own personal financial situation (and therefore what you may expect), I am in-state and have taken out nothing besides Federal Staffords and the few grants/scholarships they throw my way (as well as a tiny bit of work-study money). After tuition/fees are taken out of that amount, I end up with a refund of about $10-12,000 per semester. The vast majority of that then goes to cover my mortgage and living expenses. I have a modest grocery budget. I "go out" and drive/spend on gas more than I should, although my car insurance is very manageable.
First year, my budget was perfect. I had very few problems and even ended up with a small "cushion" after the end of the semester. Last summer, I worked for several weeks and ended up with about $3K in stipends. Not much, but every little bit helps. Started off second year with a little excess in the savings account, and then got my first disbursement. First semester went off without a hitch - then all hell broke loose.
At about the time of my second disbursement (December/January), my car died on me. I had to buy a new one. Luckily, since I have good credit I was able to get what I wanted in terms of financing, but it means an extra $200/month that I really wasn't counting on. Of course, my car insurance premium skyrocketed to about 3x what it had been before, and the new ride guzzles gas (I opted for the mid-size automatic, unlike my old compact 5-speed). Added to that was the cost to update my roof (set me back about $1600) and, a few months later, some major plumbing/electric work in my basement (another $1000). I'm scraping bottom at the moment - I wish that I had borrowed a little extra in the form of GradPlus. Sure, the higher interest rate sucks, but it's really not worth all of this worry, especially at a time when I should be focused on other things (I take Step 1 June 21st, but don't get my fall semester disbursement until late June).
It will also depend on where you plan to live, what your lifestyle is like, etc. But I lived pretty comfortably, until now. Nothing extravagant, of course - but still the occasional night out, shopping trip, etc. If I could do it over again, I would err on the side of overborrowing by $2-3K, just to have that cushion (in case of an emergency).