Ok it may not be cheaper to make a new show i am just guessing that but doing a remaster of babylon 5 would be utterly, crazily expensive, the problem is you are having to recreate existing shots, matching them to the audio, as i doubt none of the audio files and scenes for the originals still exist other than in the final product.
So say recreating the space battles of Severed Dreams would take longer to make as you are trying to get the new shots to match the originals, your having to do it all from scratch, which takes longer to do and thus costs more money for the VFX team's time.
Now 20 million to do TNG, which had less CGI and VFX shots. To Redo the all the VFX and CGI, Interior CGI on greenscreen of babylon 5, bearing in mind that in TNG a lot of the space shots where model shots so on film and easier to make HD. Babylon your recreating everything for the models from scratch. So i would expect the cost to be nearer the 30 million mark.
Would you fork out that money on a remaster project that realistically, based on star treks remasters you would not make that money back or come close. ST TNG is a very popular show and its remaster totally bombed in sales.
Also a new show may not have as much CGI, depends on the story JMS is wanting to tell.