It is in fact a house edge, you can call it what ever you like, but these fees are so high that professional traders are probably unlikely to want to use the service. For me, the fees are far to high for me to use it, although I would definitely use it if reasonable fees of 0.1% or so were levied.
I still don't think you quite understand how this works... All you see is that you are "wagering" 1 GB, but will only win 0.8 GB (total payout of 1.8GB)... and somehow you think that this means you're paying a 20% "fee".
You're not, what you're essentially doing is playing "dice" with a 1.8x payout setting.... which on a lot of dice sites, would equate to a "chance" of around 55%. The thing is, YOU need to figure out what you think the actual odds of the "up/down" is and whether or not the payout you're getting matches the "odds" of the result you select.
The "odds" are not as clearly defined as they are with dice which operate in a "fixed" environment... you're basically just guessing that the price is going to be higher or lower after a set time frame... now, if the market you're wagering on is "mooning"... then "UP" is more likely, the return will likely be reduced to reflect those odds.... likewise, if the currency is dumping... then "Down" is more likely and the return is likely to reflect those odds by being reduced.
if the market is v^v^v^v^v^v^v^v... then it is a lot more difficult to guess... and I'd say that the return is likely to be closer to 1:1 to reflect that... of course, it never WILL be 1:1, as the site owner needs to make some money to cover costs...

Payout
The payout is the total paid to the trader in case of a winning contract. Payout = stake + net return.
Example: A winning contract of 78.9% with a 1GBytes stake will award you a 789 MBytes net return and generates a payout of 1.789GBytes. The net return is fixed upon contract purchase. Its value is function of the selected underlying asset, the contract duration and the market conditions at the time of the contract purchase.
As I suspected, it is just like a bookie creating odds for sports...