Heißt das nicht sowieso eher "meinen Senf dazugeben"?

my twopenn'orth (my twopenny worth)

I'd be careful here. People (at least in the US) who say "I'll throw in my two cents worth" don't seem to regard their contribution as 'unbedeutend'. I hear the English phrase as a mixture of irony and false modesty. Anneli's suggestion sounds promising if it has either of these elements.

I guess the meaning depends. If used as suggested by Roger ("I'll throw in my two cents worth."), it means indeed "Ich geb jetzt auch meinen Senf dazu." But more often you find it below the statement given, saying "Just my 2 cents worth.", and this means "Nur meine [unbedeutende] Meinung."

(Wobei "unbedeutend" eines Hauches Ironie in beiden Fällen nicht entbehrt.)

Ich schlage vor, dass LEO jeweils die kompletten Sätze aufführt.

I woud say the singular in LEO is indeed correct. And I checked Google:
"My two penny worth"/"My twopenny worth"/"My twopennyworth" get almost as many hits as "my two pennies worth" /"my twopennies worth"/"my twopenniesworth".
"My twopennorth"/"My twopenn'orth"/"My two penn 'orth" together also get as many.
Phew! Looks like you can feel free...

"In my humble opinion" trifft "meine unbedeutende Meinung" vielleicht besser. "My two cents worth" ist meiner (unbedeutenden) Meinung nach "meinen Senf dazugeben".