How much money would I have to spend to play in tournaments?
The easiest way to compile the information you're looking for that I know of is with tcgPlayer's deck search:

http://magic.tcgplayer.com/db/deck_search.asp

Choose a "Format" 

Choose "Places Finished" : 1st, 2nd, 3rd-4th, 5th-8th

Choose "Deck Origination" : Event and Article Decks Only

Only took me a minute or two to compile that for the most recent star city games legacy open the top 8 decks had a min price of $1100, max of $2500 and median around $1600.