A roof, furnace, driveway eat up almost all of your gross income for a year, nevermind paying for taxes, insurance, debt, maintenance, etc..I wrote a blog post that might help you understand the numbers.... in fact, that's part of the title.  

Using  a Roth solo 401k to invest in real estate is a very lucrative way to grow your nest egg tax free because all gains grow tax free.

That could increase your holding costs and eat into that profit as well.An old investor I used to work with would say "the juice ain't worth the squeeze" and for $120k you might as well do some much less risk SFR flips that have a much bigger pool of buyers and will easily sell in a month or less.

Every time I sell a home the commissions are eating up a big part of the total profit I want to buy a home doing my own contract directly with the seller.

I have a retailer on my team in the South Dakota area that Has connections but I don't want to put all my eggs in one basket.

What is eating up most of your budget?  

While same could be said about how people calculate Vacancy Factor and Maintenance Reserves, I notice that people struggle the most with Cap-Ex Reserves.This especially becomes a problem when folks rely primarily on their rental income to eat and live.

If you start to budget $2K (for round numbers) for a round-trip flight/car rental/hotel/etc. you can eat away (on a percentage basis) at the nest egg pretty quickly.  

In that case you have an alligator that eats your lunch. 

A condo will have a monthly Condo Owner Association (COA) Fee which will eat into your cash flow.