flight + place to stay + steemfest ticket + some per diem for the days outside the main conference days for lunches & dinners.

You can make it as expensive or cheap as you want:

1. Take a flight or hitchhike
2. Stay in a hotel or airbnb (or share a hotelroom - see #steemfest channel in steemit.chat) or go for couchsurfing
3. Steemfest ticket, depending on your feel and how steem has treated you pick a minnow or more expensive ticket.
4. per diem for additional days in lisbon - portugal is cheap so you can be done with that very easy.

would be cool to welcome you in lisbon!