I just saw this
story which shows what broadcast channels are getting per 30 second spot for each show. It should give a good indication of what can realistically be expected in terms of revenue for a show. It looks like roughly each viewer is worth $.01 for each 30 second spot. Fringe currently makes $120k/commercial. Assuming 36 commercials for each show that means the network takes in $4.3m/episode. Factoring in costs and profit along the way Fringe probably can cost a max of $2m an episode to be profitable.
If a Star Trek TV show plans for roughly the 10 million viewers Fringe gets, then it will need to keep it's budget at $2m/episode. If it goes the cable route and only plans on 5 million viewers then it will need to cut its budget to $1m.