@Christos Philippou I use cap rates to help compare 2-4 unit properties. The comparable sales approach can be tough b/c sometimes I can't find comps, especially in Philly where each property and each block is its own unique and special flower. :-p Ultimately I look at ROI/cash-on-cash returns, but that depends on how the financing is structured, whereas cap rate lets me compare just the factors of the properties themselves.
As far as your computation, to compare apples to apples it should be consistent with what other investors standardly do, so needs:
8% vacancy loss rate, computed from your gross income
6-8% management fee, computed from gross (since like me you self manage, consider this a salary you pay to yourself for your time managing it and separate it from the property #s)
Rental license fees (if Wilmington requires this, ours are $50/unit/yr)
Trash removal (again if Wilmington charges)
Snow removal
Landscaping (cutting grass or bushes if you have any)
Insurance: Are you really only paying $750? I'm paying more on a basic duplex but maybe Wilmington is different. I'd check what values they used to come up with your premium to make sure they have correct square footage, number of units, materials etc, and that you agree with their total replacement cost.
Repairs & CapEx: I've been told to use 5% of gross income for repairs and 5% for CapEx (replacing appliances, roof, etc) and that's pretty consistent with my actual experience so far. Combined that's right around the $3K you estimated for repairs
RE tax and utls you got