Smooth on is very popular in the US and so is probably pretty affordable.
If you're overseas, then it probably wont cost you that much more for actual mold making silicone. If you're in Australia, like me, then you'd know that getting RTV here means spending hundreds and hundreds of dollars for quantities that cost under a hundred as caulking silicone.
There are tricks to using caulking silicone for mould making, you just have to understand the science of how it cures. As Dave pointed out, it cures through evaporation. It actually takes moisture out of the air and uses it in a chemical reaction, resulting in the production of acetic acid as a by-product. This is why caulking silicone smells like vinegar while its curing. Because vinegar IS acetic acid.
Knowing how it cures you can quickly work out why is doesn't cure in thick layers. Quite simply, because the lower layers don't have access to moisture from the air. Which is where the solution comes from.......adding moisture.
There are two main ways I've experimented with in the past that both work, with their own set of drawbacks.
The first..........
Take a bucket and fill it with warm water and add a decent amount of detergent.
The detergent will stop the silicone sticking to your hands.
Then take a few tubes of 100% silicone caulk and squeeze all of the material into the bucket.
Makes sure your hands are covered in soapy water and then work the silicone under the water into a ball, working moisture into the silicone mass.
Then apply the silicone straight to a sealed, smooth item you wish to mould.
You have to apply it in such a way as to avoid trapping air underneath, and you have to work quickly as the mixture will start to cure rather rapidly from the warmth of the water and the high amount of moisture now in the silicone.
This method is ok for quick, dirty, throw away moulds, where you just want to copy a rough shape for example, that will be later refined. HOWEVER, small bits of cured silicone that hardened before you applied the mixture to your item may ruin your detail.
The other method...........
Take some caulk and squeeze it into an empty soft drink bottle.
Add some mineral turps (known by a hundred other names, so do some research)
Add a few drops of glycerine, and a few drops of acrylic paint.
The glycerine helps to start the reaction, as does the water based acrylic paint. The paint also makes your mould a pretty colour. Win-win.
Then simply shake your bottle to mix the ingredients.
The mineral turps is a solvent and will dissolve your silicone into a liquid like consistency. The more turps you add, the thinner your silicone, and the easier it is to pour and pick up nice details in the mould.
HOWEVER, the turps will evaporate! This means that the mould will shrink! More turps, more shrinkage, less turps, less shrinkage.
But this method should get you buy for something that is single use, or isn't too important. The longer you leave the mould, the more it will shrink, so don't wait.
Of course, neither of these methods are as reliable and produce as good a result as proper RTV or platinum cure silicone, but depending on what you're actually using them for, can be an easy, cheap alternative for only a few bucks.
Normal disclaimers apply. If you ruin something, or burn something down using these ideas, its not my fault. Always TEST FIRST in a house that you're allowed to burn down.