Classic Backpack Cross Europe

Okay, so I've been inactive around here on a while, thus resulting in myself losing my password/user. I've already planned a extensive trip to Europe with my Boy Scout group, which we leave next week. But as always, you can't leave without already starting another travel plan.

Currently right now I'm looking ahead to the summer between High School and College. For me and my friends (5-6 of us), how feasible is it to do a backpacking trip from Spain to Istanbul across the entire central part of the continent? How far can we get for under 2000$ for the cheapest means of moving around.

I want to see as much as we can in roughly 3 weeks for under 2000... Any ideas? Is it possible at all?

3 weeks to get from Spain to Istanbul? Impossible. You'd have to be really rushing it and see almost everything flying by from a train window. 3 weeks are a good timeframe to see one big country (France, Germany, ...) or maybe 2 small countries that are close together (like Netherlands and Belgium).

Cheapest mode of transport is cycling, taking a tent - downside is that you cannot see as much.

2000 USD is a good, but tight budget for 2-3 weeks in Europe if you can get cheap tickets for under 800 USD return. It would depend on the exchange rate of the EUR to the USD and the oil price at the time. At the current exchange rate I would try to have around 1600 USD plus the cost of the flight ticket for 3 weeks in Western Europe.

It's not such a difficult one - really. A few train changes for sure. The best route (as an idea) would be Madrid, Milano, Ljubljana, Zagreb, Sofia, Istanbul. You could (with overnights) do the complete journey in about 4 days, or thereabouts. I've done Budapest to Athens on the train(s) and with a change in Zagreb and Thessaloniki, in just over 2 days. Your trip from Madrid to Istanbul is roughly twice the distance, but the train to Milano will be much faster than the steel-wheeled snails that I was on. Pack a blow-up cushion otherwise you'll have square bum by the time you get to Turkey.
Sounds like a great trip.