If you do the math out, continuous Speed isn't something that you need more than a dozen or so rounds/day, especially since Speed doesn't give you any of the other benefits of Haste, such as the move speed increase and the +1 AC bonus.
Weapons are expensive. A +4 weapon (+1 speed) costs you 4^2 * 2000g, or 32,000g (plus tax, title, and masterwork costs). Plus, it makes adding OTHER enhancements to the weapon even MORE expensive. If you want to add Wounding on top of Speed, it'll cost you a whopping 40,000g additional (72k - 32k = 40k). Because of this exponential cost of increase, you generally want to only put effects on your weapon that you can't replicate via other means.
Boots of Speed cost 12,000g and give you 10 rounds of Haste per day at no activation cost. If thats not enough, you can add Speed to your armor for like, +4000g (static adder, rather than plus equivalent) for an extra 3/day uses. Thats 10-13 uses per day, which is 3-4 uses per combat for 3 combats per day. Remember that you only really benefit from Haste on rounds that you can full attack, so its not useful on rounds you spend activating magic items, initiating standard action martial strikes (or using standard action feats), or charing without Pounce. If you sit down and math it out, thats plenty enough uses.
You could even buy 2 sets of Boots of Speed for less than the price of a single Speed weapon to get 20 rounds of Haste per day, as long as you have time to swap boots between combats. Thats a LOT.
So, as you can see, it costs a LOT, primarily for the fact that you can use it all day. But the truth is, you'll seldom need to use it all day, and the amount of time you get from lesser, cheaper items is PLENTY.
Heck, for the price of a +1 Speed weapon, you could buy a +1 Wounding weapon AND a pair of Boots of Speed and STILL have 2k left over for potions of Flying and other swag. Its just not worth what you pay for it.
