3.5% gets us mars.

Let's kill one myth to start... NASA needs more money. Quoting from the book I may never get published...

"Back in 1973, the total cost of the Apollo program reported to Congress was $25.4 billion."  Annual inflation from 1973 to 2015 was 4.15%.
(See http://www.dollartimes.com/calculators/inflation.htm)
That's $140 billion in 2015 dollars. 

NASA's budget today is over $18 billion per year which is $180 billion per decade vs. $140 billion for Apollo.

So how much would it cost to establish a colony on mars?

No matter what the plan, it should occur in two phases, the first could start today.

1) Cargo phase. Pick a spot and send 40 tons of presupply there. 10 tons being food. During this phase we are testing our human landers (Dragon version 3) Should take 6 to 10 years at $625m per year (6 or 7 FH per launch window. 2 tons per reaching the surface.)

2) Human phase (only after production rates have been established for water, breathable air, [methane] fuel and electric power and landers have a heritage) we send convoys of Dragon lander (2 crew per lander) with inflatable space at $200m per FH. Those inflatables are stored for landing to become farms and habitats on mars.