Here's what I found.
The following figures are for a single, 800 USD payment made from the USA to a UK bank account. The money is moving from the 'buyer' to the 'seller'.
* Option 1 : L$.
** Buyer uses the LindeX buy page to buy 800 USD's worth of L$. Ends up with 219458 Lindens.
** Seller gets L$ and puts them on the market for L$1 per USD less than what the lowest L$/USD conversion rate is right now. Example : if when selling L$ right now, you can get at most one USD per $275 Lindens, the seller puts his/her Lindens on the market for 274/1. They sell in a few days (unless the market is on the up and doesn't fluctuate), netting $772.90
** Seller pays L$ a 15USD cheque fee to get a cheque for $757.90 posted to him/her in the UK.
** Seller waits for cheque to arrive from USA. When it does, they cash it into their bank account.
** Out of 800 USD sent to the seller, $757.90 arrives. That means it costs 1-(757.90 / 800 )= 1 - 0.947 = 0.053 = 5.3% in fees.
** Total fees : approx 5.3% of money sent.
* Option 2 : PayPal
** Buyer sends 800USD to seller via paypal.
** 3.9% cross border USA->UK fee is applied.
** 2.5% currency conversion fee is applied.
** Total fees = 6.4%
Verdict: sending money via L$ costs 5.3% in feeds, but sending money via PayPal costs 6.4% in fees.
Notes :
1) I used a PayPal business account for the math here. Fees may be different for personal accounts. The reason I did this is that IMHO, having a business name, instead of your personal name, show on things like credit card statements is both better for your privacy and also better for your professional image.
2) Note that many of the fees here are based on the fact that it's a USA->UK transferral.
Doing the exact same calculations again, except without USA->UK fees and currency conversion fees, we find that L$ has a 3.4% fee rate, and so does Paypal!
3) Note that British and other banks may (and usually will) charge a currency conversion fee for handling forign exchange. However, you can avoid this fee by exchanging money at a post office, for example, or depositing the money into a US-based bank account.
4) Note that PayPal is *arguably!* more convenient than L$ as you can do automated billing with it.
5) Note that PayPal also has a huge website full of disgruntled ex-customers who are furious at how badly PayPal has treated them. So, they have a much worse reputation than LL in terms of trustworthyness.
Verdict : For payments to residents outside the USA, L$ is just over 1% cheaper, fee wise... assuming that your bank doesn't charge a currency conversion fee. For payments where both residents are within the USA, the price should be around the same. In this case PayPal might be a better idea as with PayPal you can set up automatic, reoccuring payments to automate the billing process. Unfortunately, this feature doesn't exist in SL yet *hint to devs!*.
Please comment on this. Math isn't my strong point so I've tried to show working here. I don't mind if I turn out to be wrong, but I would really like to know how the math stackes up in terms of which method is cheaper for getting payments

If you can think of cheaper ways of accepting payments, let me know. Similarly, if my math is wrong, let me know
