the most rare items are the:
Promise ring ($1,000)
Weird alien ($600)
Combat copter ($300)
RC helicopet ($200)
Mario and Peach kart ($100)
Meteorite($150)

cheap right? well...for me $20,000 is very low.
wanna get lot of money?
Get a Labrador Retriever and train it so it is easy to win championship. do 3 contests and win $3,000, then save, turn DS off and go to setting. set it like this:
day before and hour 23:59. go tu Nintendogs and wait for 1 minute. you will be availbe to do 3 contests again.

Note: while walking get all "?" boxes you can, then to the park, and then to your house. when you at the park, change accesory, go back and game will be saved. turn DS off and on. you will be in your house and you can go for another walk!.