SWAPS are typically a function of interest rate differential. Based on my analysis there are two types of brokers. One that focus on commissions and let the swaps be calculated theoretically and another types who focus on swaps and cover that up in their commissions. Technically, I prefer the first type but it also depend on your strategy. If you are a low frequency trader, you should focus more on swaps and not as much on commissions. While if you trade more than a couple of times a day, you should focus more on commissions (this , mathematically is irespective of the size of the trade but the average size of the trade)
Here is how theoretically SWAPS should be calculated in a “no-arbitrage” pricing model.
(Interest Rate of Numerator - Interest Rate of Denominator ) / 365 (or 252 based on frequency of swap posting)
So for example, if you are long BRLUSD and assuming the interst rate on BRL is 13% and that on USD is 1% then your swap charges is 12% per annum. So for one lot of BRLUSD you will pay 12000 USD per year or $32 per standard lot per day. And if you are short BRLUSD, you will earn $32 per standard lot per day.
Now to make it precise, you will have to add interest rate factor of your account into prespective. This is a hidden area and every broker has his own interest factor. But this in theory is to cover the 400X leverage that you are utilising and the cost of providing you with that leverage. Lets say if the interest cost if 1% and you are using 400X leverage, then this cost is 4% per year for the broker and your swap rates for Long BRLUSD will be 16% (13% - 1% + 4%) and Short BRLUSD will be -8% (13% - 1% - 4%) which means that you will pay $43 per standard lot per day and if you are short BRLUSD you will get $22 per standard lot per day.
Also note, there are only 2 variable factors in this One) What is internal leverage cost of the broker and Two) is the broker baking in swaps in the commission or not (lower commission versus higher commissions)
Now these examples are in no way reflection of Finpro or turnkeyforex or turnkeyfx but a general an example that is used by each and every broker in the industry. I know it so precisely because I used to do swap arbitrage and this is the basic calcuation that needs to go into the model to find arbitrage opportunities. And if you have an interest in SWAP arbitrage, you should try and find islamic accounts and see how you can play around that (sorry, no more details on a public forum about it )
Hope that helps