Visas and border crossing payments can add up very quickly and on some routes can become the largest chunk of your spending. So if your budget is tight you'll have to carefully balance your hit list of countries against the risk of going destitute.
While undoubtedly it's impressive to your mates that you've been to more countries than they'll have heard of, transiting a country in a few hours (as some of them will have to be) seems to us to rather defeat the point of looking for adventure along the way. For every desert or mountain range you're team is eyeing up there's an equally stunning cheaper alternative.
As a rough guide visas and border payments per person will roughly add up to £200-£300 on the Northern route, £600-£1,000 on the Central route and £1,200-£1,500 on the Southern route.