As to the relative values of tossups vs negs, it
depends on the team you're playing more than anything.
For a set of variables, let q be the probability that
if you wait until the end of a question your team
will get it. Let B be your team's bonus conversion,
and let b be the opponent's bonus conversion. (For
simplicity, I will assume all questions get answered if read
until the end, even though this may not be the
case).
A Tossup is simply worth 10+B points.
I
would peg a neg as worth worth (-5)-q(20+B+b) points
(the second term comes from the q proportion of time
your team would have gotten it right and converted B,
but instead the opponents got it and converted b).