$10/mo for an additional line if you need it, then $30/mo for BIS and I think it's $45/mo for BES, but I could be off slightly. Then of course there's the cost of the phone.
Go for the one closest to college age, the father of four speaking!!
The BIS plan is basically $30 per month and it will be another $30 per month if you want to use it as a modem by tethering. The terhering part is of course if you want to comply with the Verizon policies.
Just a little note here nobody told me about. I was on the premium plan (just me so not sure how it would affect others), and when I got the storm they dropped me to a lower plan because some of the features I used from the premium plan were on the BIS plan. So to add a $30/month feature my final bill went up $6/month. Call Verizon and have them look at your plan & features. You might be as pleasantly surprised as I was!