In Dexter, Season 4, Dexter keeps two houses, one of his old one for secret works and one for Rita and his children. Similarly in Season 5, He returns to his old house but still keep's Rita's house for Lumen. So is this possible in Miami to hold two houses on rent on the salary of a blood analyst? Dexter has other expenses too like maintaining killing rooms/equipment/killing costume , expensive cars,boat, nanny's salary etc. etc.
I've found information (Source) on the net containing salary ranges for "Blood Analsyts".
In addition to this, Dexter sold his biological fathers' house in season 1, from which I would guess he would get at least $100,000 cash in hand.
I also noted a comment passed by Angel Batista in Season 2 when they were trying to catch the "Bay harbour Butcher"
Which is where Dexter kept his boat, while not hard evidence, but does conclude that where he kept his boat is one of the cheaper harbours.
Lastly, while conjecture information, up until Season 4 he wasn't the legal guardian of Ritas' children and had no dependant children himself yet, so when he did have dependant children, I assume he would be eligible for government paid tax credits. Not being from America, not sure of the laws for that