A quick telephone call should get you the tournament details.
Things to consider from my perspective:
Re-buys cost
Add-on and cost
Equity
Distribution of prize pool (How many places, or percentage of field get paid)
Anticipated number of players (History of event from years past)
WHO received invitations? Will I possibly find myself in a field of 10 with the other 9 being the likes of Brunson, Hellmuth, Negraneau, etc?
I probably would not pass on the invitation if the prize pool paid out about 10% of the field, and I was "there". After all, it is a freeroll, and there is no requirement that says a player has to re-buy or add-on.
I do not play a lot of tournaments, although I do seem to hold my own when I do. I rarely buy in for full price, but will purchase entries at significant discount or when I win them for "high hand", etc.
Did make the money in a freeroll in 2010. Small amount, but worth it. I was the only player who cashed without making a rebuy. (I only played 4-5 times) I will not rebuy, but will purchase the dealer add on chips (staff tip) at the tournament start.
IMHO, it's always worth a shot, if not too "expensive" to attend. And, the cash games during tournaments can be really juicy. THAT is my primary reason for going.
Good luck.
btw: Many "Guaranteed Freerolls" with re-buys, will not return the excess of the guarantee until they re-coup their normal tournament fees. So if there are excessive rebuys, the prize pool may not increase. It's important to find out the terms and conditions, etc. I'd call the room and request the "FAQ" sheet or a link to it.
HTH