If you need to save money, then buy used... if you buy a RTR, then you will buy twice.
Up front costs to get in the door are a drop in the bucket... be sure to factor in the race fees and tire wear, those are the most expensive aspects of the hobby. Ask locals at your club for which classes are the least expensive to run on tire wear... my club runs on turf and the 2WD buggy class tends to be the most expensive with tires only lasting about 2-4 race days.
Spec Slash is the least expensive to get into the hobby at $200 as RTR with only battery (LiPo), servo and radio to replace... then tires typically last around 20+ race days before they need to be replaced as an example.