Are you buying swaps or futures on BitMex? With swaps you'll probably have to pay funding, but with futures you won't although they will expire and you may need to pay a premium (sometimes you can get them cheaper if the market is falling). Also since the funding rate largely depends on market conditions, it is hard to predict. At the worst case in a bull run you'll be paying 0.375% every 8 hours which is a lot for a "mortgage". Granted things could reverse and you could get paid the same amount back but that is rare and overall longs usually pay more funding.