Monthly Electrical Cost incorrect

From my understanding of the rate structure you are using, net metering does not apply, so I think it is probably a coincidence that the costs are close to what you expect when you enable net metering in SAM. Also, there is a bug in the net metering calculations in SAM that results in the sales/purchases value for January being incorrect (See the Download page for a list of known issues). We plan to release an update to fix this and the other issues very soon.
What is more likely to be causing the difference you describe is the "Max Usage kWh" values for the tiered energy rates. In SAM, that is a maximum hourly value. That means that in your file, the Tier 2 rate will only apply in hours when the hourly kWh from the grid exceeds 675 kWh.
I've attached a spreadsheet to your original post that shows that SAM applies the Tier 1 buy rate ($0.1323/kWh) to all hours of the year. That's because the hourly grid purchases over the year range from zero to a little less than 12 kWh.
Is it possible for you to recalculate the Max Usage kWh values for each tier to be maximum hourly values?
Best regards,

Paul.

The spreadsheet should be attached now -- sorry about that.

I'll have more on this after I've consulted with the developer.

Thanks,
Paul.

Yes, your understanding is correct. For the next version of SAM (Fall 2014), we plan to fix the tiered rate model so that it accumulates kWh's over the period you specify (Period 1, 2, etc.) instead of hourly.

Best regards,
Paul.