Sta.sh
1st Place 
I know by this happening exactly who I can always count on, and it is an amazing feeling 
400 DA Points cost US$ 5 to buy.
That's 80 points per dollar.
This means that if you are offering an art commission for, say, $10, you should NOT be offering the same commission for 100 points. You should be charging at least 800 points.
If you are aware of the exchange rate and yet still undercharge for points when compared to real dollars, I suppose that's your prerogative as an artist. But you're really just giving others a way to take advantage of you by paying you in points instead of cash, even when they are fully capable of the latter.
Those looking to commission artists with points should, of course, be aware of the exchange rate, too. I have seen many people complain about artists asking for 300 points for their artwork... not realizing, of course, that it's only $3.75!
And, of course, the same applies when paying in different real currencies around the world. Make sure you check your exchange rates.
Financial matters matter.
MSW
EDIT: ~MaureenEileen pointed out this lovely DA points to US dollars converter:
Very useful if you are still confused about the exchange rates or just plain don't like doing math.
The only response I got was that "they cost a lot for just an icon." (just an icon?
On the other hand, there are artists who charge their commissions in cash only but people still offer
Anyway this place is oversaturated with kids without paypal/bank/credit/debit card/money and artist without money or maybe some cheapskate adults/clients from company so it's hard to get a well paid commissions unless you're popular. But even the popular people find it hard to get commissions in DA.