Two thoughts: First, since the rights you'd be giving away are in perpetuity, just take the amount you received for the first article...and multiply it by 50% of eternity!

Second, report this dolt to the shop steward at NWU...

Seriously, ask for what you truly feel it's worth - as you know, it's hard enough to try to make a living from wordsmithing without selling yourself short! Good luck and continued success to you.