I have read a lot of confusion about this subject and would like to clarify the numbers as some others may already have.
Freerolls are free to play and real money to be won. Besides money, there's also experience to be gained. Easy as that. It doesn't matter how much. So the ROI is infinite really, cause the investment is nil, therefore the return is whatever you gain.
Now, if you know you're good and you know what you're playing and you have the bankroll to spare and play with it, then play for whatever you can afford and grow your money accordingly. But that's what they call a poker player not a hobbyist.
However, if you knew what you were doing and had the capital to do it, you would not be here asking this question.
My advice is play as many freerolls as you can until you gain the knowledge, experience and bankroll to be able to advance in your chase of the poker career. Even more so, play freerolls if you're simply playing poker for fun because you're not losing your personal money. Simply because if you were just confident in your abilities (that you probably lack) you would go ahead and lose your money, just like in trading for example. This is not an activity based on feeling but it's rather based on quick mathematical decisions. 1 extra point for ploaying freerolls, without wasting your money.
As in every single job you get, start this poker job from the bottom, as in grind the toilets with a toothbrush first before you start having a clue what this "poker" job is and you will eventually advance in your poker role.
I see many people getting upset because they lose with AA 5-6 times in a row or their KK don't hold, or the straight turns out to be the loser on the river. Now, this is poker, it's a long-term job, not an instant gratification scheme. You play AA 100,000 times and you will win about 70,000-80,000 times. and then you notice the difference. If you haven't even played 100,000
hands in total, you are not entitled to judge those few AA you ever played.
Poker is a big picture, look at it whole and put in the effort. Or don't. No one makes you. It's your decision.
P.S. If you play those Aces badly everytime against multiple opponents in the hand, then your win percentage drops severely and you'll say it's rigged.