Small tank has small tank fun, big tank has big tank fun..... The setup cost varies depands on your tank size, the equipement, substrate etc can can be from less than a hundred(entry level + DIY stuffs) to few thousand(all automatic and changgih stuffs)....

To start off with, I guess a 2feeter will be best. Once you get use to the skill, then you can upgrade to biger tank and more sophiscated stuffs...
LFS(Local Fish Shop) will be the best place to start off with to get your needed equipments, if you don't have any other means of getting those stuff. DIY also a good idea as part of this plant tank hobby.

For planted tank, beside the tank, light, substrate, you also need CO2 set(tank or DIY), filter and other accessories(eg fert-later on, driftwood/stone-for scaping your planted tank-aquascaping, termometer-to monitor the temp and etc).

Surf the below page for more 'Get Started' Info....
http://aquaticconcepts.thekrib.com/