I have a big ol head, too. I wear a 7 3/4 or 7 7/8 motorcycle helmet, and Giro bicycle helmets are the only ones I've tried that actually fit. I hear the Limar lids are large, too.
To the OP, you can sometimes find non-current helmets on sale for huge discounts.
It's your head, and I wouldn't try to tell someone to wear one, but if you smack your cavesa on the asphault, you'll wish you had one on. I had a pretty bad concussion, and always ride with one now. I have broken two of them, and had no head injury either time. I don't know what would have happened if I wasn't wearing one, but I'm glad I was.