I don't know what you paid to rent the tiller but around here it 75.00 to 100.00 per day. You can run an ad on craigslist or look on craigslist and get someone with a small tractor to come till it for you for less money than you pay to rent a tiller. I tilled gardens for 3 years when I got my tractor and PTO tiller. I paid for the tractor and tiller using it to till for other people. A garden your size would only cost $50.00 and I could do it in 15 minutes. I would till to at least 8" This maybe something to look into this year, If not next year for sure. It could be cheaper than renting. Let someone do the hard work for you with a tractor. Here is one in Brainerd They use a tractor with a 5 ft tiller >>>
http://brainerd.craigslist.org/fgs/5543999475.html
Here is one in Bemidji they use a rear tine walk behine tiller>>>
http://bemidji.craigslist.org/fgs/5488062174.html
Here is one in St. Cloud>>>
http://stcloud.craigslist.org/fgs/5530653225.html
Just look under Services then under Farm+Garden