ok this is simple my friend and i worked it out simp algebra
just enter this into your calc
3^(N-1) where as N is the + number
for example you want a +4 like i have on my coro
i enter
3^(4-1)
or simp
3^3
which is 27
and then 27 X (price of +1 coros) (650k on my server) is your answer so my coro that elite +4 i would sell for 20-21kk
for you who don't use math the ^ symbol mean raised to the power of
but if your really lazy my friend uses this site
*i take no credit for these discovery the math was my friends idea and i give him all the credit and as for the website my guildie gave it to me
happy composing