Duck,
Type ratings are good for as long as you have your certificate. There are certain training and recurrency requirements dictated by the FARs regarding sim or in aircraft training in order to utilize the type, but otherwise they don't "expire" like a CFI might.
IMHO, there is TYPICALLY no need to purchase a type rating. I am a firm believer that under usual circumstances an EMPLOYER should pay for the necessary training. That said, there are times when obtaining a type rating on one's own is appropriate (I have done so myself).
Transition training between similar types depends on the type rating and the manufacturer. For example, the 737 type covers all derivatives of the plane from the -200 to -700/800/900, but the CRJ type rating is entirely different from the Challenger 604 which is almost identical from a systems standpoint (minor differences.) The CRJ type though is good for all of the CRJs (200/700/900) with difference training.
Some aircraft transitions allow/require short course training (such as one week), differences training (1 day + 1 sim), or the full course (14 days - 1 month)
I would say that unless you have an ATP (or enough hours for one), and several thousand hours (say 2500-3000TT and 1000PIC), the cost of the training will most likely not outweigh the benefit.
There are certain employers that require types before employement (such as Southwest and many corporate jobs). Whether or not it's worth shelling out the money is up to the individual pilot after conducting a personal cost/benefit assessment.