I've done a quick bit of research on the various exchange rates offered by the above entities, and was a bit surprised by my findings, so I thought I'd share with you guys for you to be aware of, and/or use to your advantage.
Firstly, many of us use xe.com or google.com to get the currency conversion rate of the moment. I'm pretty sure this is an average of the wholesale buy and sell rates happening around the world at that moment. This is never the rate you get.
The next rate that many of us (when dealing with eBay) is eBay's built in conversion estimation. For those that don't know what I mean, if you are looking at a foreign auction at a foreign eBay, i.e., ebay.co.uk, you can replace just the ebay.co.uk with ebay.com.au leaving the rest of the URL alone, and eBay will show you an approximate AUD amount under the native currency. I have found this rate to be the most accurate, but consistently slightly low.
Now, when you go to pay for an item, you can use PayPal's conversion rate, which they describe as 2.5% above the standard wholesale rate. On their fees page (link requires you to be logged in), they mention this and give you a convenient link to their own Currency Converter. They even present the result as the "Paypal Exchange Rate". This rate was lower than XE in some cases, making it extremely optimistic, and equally as unrealistic. Do not trust this tool for any reason.
I then stumbled upon a dead accurate way of finding out the amount that PayPal will charge you. Once logged in, choose to "Send Money". On the following screen, you can enter an amount in any currency and PayPal will show you an estimate in your currency. Obviously, there is no need to proceed past this page if you are only after an estimate. Granted that you complete the PayPal transaction before the rate changes, and use PayPal to do the conversion, this is the exact amount that you will pay. I recommend this method.
Lastly, you might choose to pay the foreign currency amount using your credit card, allowing your bank to do the conversion. As STL recently pointed out, his bank's rate for doing this, as is my NAB Gold card, is the same as PayPal's 2.5%. However, when you do a credit card transaction, the money isn't actually withdrawn from your account for a business day, or two or three. You will be assessed the rate at the time of withdrawal, not time of transaction. So, depending on your rate, and the current currency trends, this might be a good option, but it is naturally somewhat unpredictable.
So, here are the numbers;
For an item priced at USD 37.50, I checked the following sources:
AUD 48.87 (.76734) - xe.com
AUD 49.42 (.75880) - eBay
AUD 47.61 (.78765) - PayPal Currency Converter
AUD 48.80 (.76844) - PayPal Currency Converter + 2.5%
AUD 50.09 (.74865) - xe.com + 2.5%
AUD 50.13 (.74806) - PayPal Actual Rate
For an item priced at GBP 17.99, I checked the following sources:
AUD 35.63 (.50485) - xe.com
AUD 35.71 (.50378) - eBay
AUD 34.78 (.51725) - PayPal Currency Converter
AUD 35.65 (.50463) - PayPal Currency Converter + 2.5%
AUD 36.52 (.49261) - xe.com + 2.5%
AUD 36.65 (.49086) - PayPal Actual Rate
In both cases, eBay's estimator was the most accurate, but still optimistic. The most infuriating one is PayPal's Currency Converter. Even if we apply the 2.5% that PayPal claims they charge, it's still grossly optimistic. Applying that same additional 2.5% to xe.com's estimates was definitely the most accurate, short of asking PayPal directly.
All of eBay and PayPal's numbers seem to insinuate that both PayPal and eBay are in the business of making you think you'll get more for your native buck. Be weary.