Making the jump from 2d to 3d

Having done a few smaller games, all 2d and somewhat simple, I am wondering, what is required to make the jump from 2d to 3d? I know there are 3d game engines that simplify the whole process, but I am talking starting from scratch. I am assuming some pretty high end math is involved for the actually rendering (with perspective, the horizon line, angles, ect) involved. Also, WHEN the jump is made, how much more power and production time is needed? You have to setup models and textures, is it even worth considering? Thanks =D

Check out 3DSTATE.

How much from scratch is from scratch for you?
I’ll assume that you want to use either OpenGl or Direct X.
You will have to know how to use matrixes and get pretty good at 3D vector math.
It also depends on how good you want it to look and how fast.
Making it look good and run fast is not an easy task.

There are a few things you need to learn about:
3d math:
Vector addition.
Dot product.
Cross product.
Normal vectors.
Matrix multiplication.

3d rendering:
Perspective.
Clipping.
Lighting (AMBIENT, DIFFUSE, SPECULAR, EMISSION).
Textures.
Texture mapping.
Radiosity,

These are only a very few things you will need to learn.
Unless you want an extensive learning experience I would recommend you use a 3D engine.