So this is the rough plan

LONDON

flight: ryanair ~50 euro

THESSALONIKI (GREECE)

train: ~30 euro

ATHENS

flight: ~100 euro

ISTANBUL (TURKEY)

train: ~30 euro

SOFIA

train: ~30 euro

BELGRADE (SERBIA)

train: ~30 euro

SARAJEVO (BOSNIA)

train: ~30 euro

ZAGREB (CROATIA)

train: ~30 euro

LJUBLJANA (SLOVENIA)

train: ~30 euro

VIENNA (AUSTRIA)

train: ~30 euro

PRAGUE (CZECH)

train: ~30 euro

BERLIN

flight: ~50 euro

LONDON

Rough cost:

flight - 200 euro

trains - 300 euro

bus+accomodation+food - 60 euro per day

that brings us about 1500 euro. so to be safe make sure you have 2000 euro