Taxi from PGIA to Corozal: I know it has been done for US$75 (a few years ago). I wouldn't expect much, if any, change from US$100.
There are a few transfer services. Have a look at the BelizeBus site mentioned in a previous post. Scroll down to the Getting Places in Belize section and you'll find a list under Transfer Services. You'll have to contact them for prices.