Undead are weird since they don't follow the same rules as everyone else for Psychology or for losing combat, as well as being able to raise models to replenish your losses. They rely on magic and either combat for VC or shooting for TK, but primarily magic and the buffs you can get from it. They're also fairly good at wars of attrition due to large numbers that can be replaced when lost and they never run away, but against a strong combat opponent they're going to lose models faster than you can replace them. Both armies are fairly slow, and rely on the General to be able to move worth a damn and/or cast magic.
I'd add a vote to this - Ogres are very easy to play (for me, too much so - I got bored of Step 1. Cast Gut Magic Step 2. Charge very quickly), although I'm not sure that they're significantly cheaper than any other race due to the cost of their characters and the number of basic Ogres or Leadbelchers you want. Ironblasters are dirt cheap for GW models though, and they're amazing, as are Mournfangs who are *fairly* cheap per point as I remember so it all depends on army build.Since you're just starting out in fantasy I'd personally recommend starting with ogres, they're tough, easy to paint, and you don't have to spend allot in order to get a large force.