could someone remind me:

if my player has 900k per season and 12m bonus each season for the next five seasons, and I then trade him....will I have to pay the bonus up front or does it become dead cap....or is that the salary Im thinking about????

Current season bonus money is already added to current year cap. All future year(s) bonus money goes to dead cap for the following year. You only pay % of salary for what player has played in for current year.

Great so I'm right to not trade this guy as he'll sting me with 12m dead cap each year

He would sting you for $48mil dead cap next year. All remaining years bonus goes to the next (following) year's dead cap. So you would have $12mil x 4 years = $48mil for next year.

Great thanks! :)