Which is more expensive, pre tensioning or normal beam and slab
In general, normal beam and slab are used for short spans because it is more economical. For larger span, other methods may be more economical, such as truss in steel structure, prestress in a concrete structure, folded plate, ribbed / waffle slab, corrugated slab, hollow core slab, box girder, cable-stayed beam, etc.
That isn’t that simple. It depends on a lot of parameters. Either can be more cost-effective under the right conditions. I will say you see more conventional reinforced structures than you do post or pre-tensioned structures. So that should give you an indication.
Like another poster pointed out it depends. For large span girders (think highway bridges ) and large span slab systems post tensioned/pretentioned systems are more economical. For smaller spans as in residential construction, of course normal reinforced concrete systems are more cost-effective.