As a rule of thumb it's a good idea to get the biggest you can afford. Also keep in mind living arrangements, etc. as a 100 Gallon is a big tank to have to move. Also consider the weight when placing it.
As for cost... just a really random guess I'd say I've got over $2000 invested in the 125 so far. You can make it cheaper but I wanted it heavily planted so went with the high output lighting and a really good quality planting substrate.
Hope this helps when deciding on your new tank.