Banking costs are a non-issue. Pay using PayPal and the currency is automatically converted. Same if you pay by credit card. Fee is no different than it is for other transactions, generally speaking.
If you enter into an inter-currency transaction where an automatic conversion is applied then you will, to one degree or another, get fleeced. Either by the transaction charge or by an overly large dealing spread on the conversion or possibly both.
The banks remain less than transparent on this one although it is a bit better than it used to be as it is now possible to get something much closer to the market rate on relatively small amounts of money through specialist brokers or alternative outfits such as TransferWise.
But for the odd, infrequent, transaction the actual difference is going to be pretty small and the convenience likely to be worth it. However if you are doing it regularly then the best way is to establish an account in the "other" currency and transfer funds to it via a broker. That also has the advantage of fixing the rate and also dampening out short term fluctuations in rates.
A: You misunderstand of the markets. They don't bet on what they think will happen in the economy. They bet on what they think other market traders think will happen in the economy.
Well, not exactly. For one thing they are mostly all using the same models and the same data so it amounts to much the same thing.
I think the reality is somewhat more complex - nothing gets a trader fired up more than thinking they have a better handle on future market movements than their compatriots so I think I would say that a trader bets not only on what s/he thinks the market will do but also on the difference between that and what s/he thinks the other traders think that the market will do.