I found my Focus with 14K miles on it for a steal (less than $8K). I am in the same situation as you. I make out like a bandit every month.
Also, most employers do NOT keep track of your mileage checks on your W-2, so the entire government rate is tax deductible, allowing you to pull in another ~$1500 from taxes (federal), not to mention filling out a Schedule C lets you make CITY deductions too. Even if they DO keep track of your mileage checks, you can deduct the difference in the gov't rate vs. your reimbursement.
TL,DR = Buy a LOW mileage car that gets good gas mileage, and buy as cheap as you can.