For the Gluten Free and the Classic plans, it's $5 a month if they pay monthly, $4.50 a month if they pay six months up front ($27 for six months), and $4 per month if they pay a full year in advance ($48 per year).
For the Classic+Gluten Free, the base price is $6 per month with a 10% discount for six-months and 20% discount for annual.
Details - http://5dollarmealplan.com/upgrade-membership/
There is an offer within the email autoresponder on day 7 that offers an additional 10% off the first billing with the coupon code FIRSTWEEK. There are no restrictions on this code so you can offer it if you think it'll entice them to join.