What is a realistic ROI expectation ?

for an ameteur ?
for a middle ?
and for a pro ?

It depends on the stake level, the number of tables in the tournament, the structure, the game, ect. ect.

I'd be interested in the answer to this too. Let's say you're average buyin is around $3 and playing 90-180 mtts and large field mtts, NLHE.
I understand one can only estimate, not knowing all the variables involved, but I imagine that there is some consensus out there regarding a generalization about expectant ROIs for amateurs, players and pros...?

my roi keeps going down as long as i play....it is hard to beat the rake at HU.

I think is best gauge is to get more than 60% itm Bottom or 50% itm mid. wheneer u wins a profit to cover the next buy in, is good enough...

two times your initial investment I think would be good ROI.

You're way off here^^
ITM is something that is much easier to give a general estimation about (you can check out a bunch of the winning MTT players online, and from a very large sample size get good picture of what an avg. itm is). The top (higher buyin) MTT players online are I.T.M. 9%-11% (more towards 9).

As far as ROI goes.... it depends upon what type of games you're referring to. For MTTs it varies greatly.... you'll see between 20 - 100+% in some fo the top MTT players.
For Sng's it's easier to give a general figure.....
Generally speaking, these days in higher buyin SNG's ($100& up), good players ROI is only round +2-3%
In mid-stakes.... 4-10%
low stakes 10-25%
.... this is for 9-plyr sngs

The good poker players who are regs. in the 45's will often have a much higher ROI, +15-+25% range ($25-$75 buyins).
Not sure what it'd be for lowstakes..... although there are some who will regularly hammer away at the $3 & $6 90's and do well.