What would be a fair asking price for renting out my parking space?

Stuff you might need to know:

I live in a townhouse. The building has 8 Units, 2 parking spaces for each. The neighbors in the Unit next to me decided they'd go ahead and get a third vehicle.

Someone is staying over for a while or something, not too sure on the details but that's probably not important anyway. So, now they're asking if they could rent my extra space. It's the only space I have available for when there's company over, granted that's not very often but still it's mine.

I was thinking possibly $5.00/Day which would be ~ $150/Month. Is this fair, too low or too high? My brother thinks I should charge them $7.00/Day but I'm just not sure...