The term penny wise pound foolish might apply here.
[Edit: Oh ... $3,200 to break the lease. I first read that as $320. Ya, man, if you are paying for the space regardless, put those free watt hours to work! Are you sure you can't negotiate some deal to get out for a lower amount?]
A 5 Ghash/s operation will generate a little over $400 per month at the current difficulty and exchange rate:
There will need to be some method that removes the heat for a 5 Ghash/s in an office area.
Also, you are assuming the landlord won't notice the $250 increase in the electric bill and that using the space for this purpose is not a violation of the lease agreement (sometimes they prohibit data processing gear or have caps on electric use).
If you had no restrictions on how much electricity you use and can easily vent then this could be doable, but then what do you do after the 8 months?
There might be a mining operator in your area that is looking to expand but is out of either physical space or electrical capacity who might be interested in subletting your space, but then for a period of just months, that's probably a deal killer. [Update: But after noticing that you are describing paying the full rental amount regardless of whether you stay or not, ... this is now probably something to consider.]