MTGO can be expensive if you are too lazy to sell your cards before they rotate out. Also, leagues are known to be the worst format with regards to "pay out" - the ratio of prizes given out versus product/tix consumed to join is significantly lower than either 8-man drafts/constructed or premiere events. However, if you are good enough, you can float your prizes and parlay it into entries into the next event and sort of "go infinite" as the expression goes.
Or, alternatively, you can skip MTGO altogether and download 3rd party freeware from some place like
http://www.magic-league.com. Back when I was a PTQ regular, I used to play Apprentice/Netdraft all the time, joining the appropriate channels in irc to find opponents. This removes the component of collecting/owning cards, you just play the game any way you want. It's optimal for pre-testing decks before buying the requisite cards. The fact that the third party software doesn't enforce the rules, or hand-hold the play experience didn't bother me, but the one unassailable feature of MTGO (besides legitimacy) is that you get real competition from quality opponents motivated to win. Apprentice/Netdraft can sort of be analogous to playing poker for play money.