Rough guess of 15-20 thousand, but largely depends on how much multi time you want, the minimum 15 hours PIC for the MEI or more. You'd need about 150-170 hours (I'm guessing I'm going to finish with around 290TT) which you can do either single or multi time. The three CFI certs would be 5-7k.
For the cheapest route, timebuild for your commercial in a single, get to about 235 hours, and save the last 15 to transition into a complex airplane, which is a requirement of your initial commercial ride. Then do your CMEL addon, which will take 5-10 hours with an instructor. At this point you need to fly a couple trips in the twin and get about 10 hours PIC. Save the last 5 for MEI prep. Then start the CFI program
For more multi time...
get a private multi license first
decide how much multi time you want and do your timebuilding in a twin or single or a combination of both. You still have to do the commercial ride in a complex, but whether you do it in a single or twin depends on what plane you want to take for 10 hours(5 day 5 night) of solo flight . If you want to do your CMEL first and you did NOT get a private multi, you can fly those 10 hours in the twin with an instructor. So here you have a choice, CSEL or CMEL initial.
CSEL = moving over to a complex single, the Piper Arrow at Skymates, and learning the maneuvers in it. say 5 to 10 hours for that. the CMEL addon would be quick (~3 hours) because you already know the plane.
CMEL first = It's the same maneuvers you did for the private, stalls/slowflight/steep turns and engine out procedures, single engine approach. For the single addon you can do the commercial maneuvers in a C172.
So for a breakdown (Assuming 110TT and coming to skymates

)
Cheap:
125-130 hours timebuilding in a 172
10-15 hours dual in an Arrow
CSEL checkride
~10 hours dual in a seminole
CMEL addon
More multi:
~10 hours dual in a seminole
AMEL checkride
120 hours timebuilding, break it up anyway you want between single/multi
At this point you have 240TT, and either way for the initial, you'll have 15ish hours dual for the both of them
15 hours dual, 10 single 5 multi
Then the CFI program. Get the rates off of the website and start adding it up, I'm too lazy. This was also to just meet the minimum requirements of 250 hours TT, you can of course, do as much as you want.