While there is time and money issues they could just creat 2 or 3 good species and use them for more than 2 episodes rather than give us 50 crappy species like we usually get.
Unfortunately, the logic of Star Trek is that at least one or two aliens will be necessary every week...at the very least, a few non-human Starfleet officers. Then there's the alien species of the week, the civilian aliens milling in a crowd....in practice, it's almost impossible to save and proportion your money like you suggest. Star Trek, in its simplest form, is basically the Alien of the Week, and in this climate, big, complicated alien species designs are very, very difficult to pull off...