Its going to be hard to give a good estimate because of location and age differences but to give you an idea...
I had a 04 GT in July my insurance was like $83/month
In October I traded the GT for a 03 Mach 1 and my insurance went up by $16/month
In November I traded up to a 04 Cobra and my insurance went up by $4.5/month
So...on a 04 GT I was paying $83/month
on a 04 Cobra $103.5/month
I pay my insurance in full every 6 months and get a discount for that...
Keep in mind that I have Progressive insurance with a multiple car discount, I'm married, 28yrs old, no tickets...yadda, yadda, yadda...
Oh and when I turned 21 my insurance stayed the same...when I got married it stayed the same (Had Geico for both)