mauritania at the border about 4000something for 10 days.
once you are in nouakchott you can buy there insurance with covers
you for senegal and gambia in advance.
ask around, not all offer cover for gambia.
don´t know actual prices, but not costly,
depends. on your "cv" ( i.e. horsepower ).
but far !! cheaper as to buy it at the border,
( and the only one office at the border selling overpriced insurance
because no competition there, will hate you
at gambia itself, insurance for one month is also only a few euro.