MS generally doesn't drop the price of their OSes when released. The price XP is at now is the same price it was at when it was released IIRC. I know the retail prices haven't changed at all. The only time I can think of when MS dropped the price of its OS was with Vista and it was a small drop in price, and it only applied to retail copies.
You're probably not going to find any good deals on XP, unless of course you go to one of those scam sites.
In which case, it WON'T BE such a "good deal" at all, no matter how low the price is.
Additionally, XP is all its iterations is currently VERY POPULAR among hobbyists, especially those who do not have powerful-enough machines to run Vista.
By the way, MIcrosoft NEVER "sets the prices for its OS". They DO publish their M.S.R.P., but NEVER require third-party retailers to abide by that price. Each retailer is allowed to set the prices for consumers according to MARKET conditions. This is why SOMETIMES you will find Retail copies of XP for MORE than Microsoft's MSRP.
Currently, Market conditions do not allow for lower prices, since the number of available XP copies is rapidly shrinking. This is because Microsoft currently has XP "end-of-lifed" at June 30, 2008. NO more copies will be available through retail channels after that, unless Microsoft changes this date.
What this means is that Windows XP right now is a "HOT" item for retailers.
If you can find such copies for LESS than the MSRP from a reputable dealer, you will be extremely lucky.
NOTE: PLEASE, oh PLEASE, STAY AWAY from those sites which offer it for LOW PRICES, such as $50. You WILL NOT be happy after doing this, I assure you. You might even wind up with your identity being stolen, your cards disallowed, and your bank accounts emptied, all because of NOT using a reputable dealer. And STAY AWAY from eBay if you want to purchase a valid license.
I ONLY purchase Antivirus technology from the former Soviet Union. Anything else is bound to be pirated.