A little clarification on magic weapon creation please

[ProsperoWilder]

I'm looking for a little clarification on the following rule....

Adding New Abilities

The cost to add additional abilities to an item is the same as if the item was not magical, less the value of the original item. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 longsword.

If I am reading it correctly it seems a little broken. A player of mine provided me with a scenario where he was going to make a +10 weapon for about 25,000 (by making it in stages "adding on" abilities one at a time instead of making it all at once) did we read this properly?
I have a house rule where you pay the full cost of any added on ability as if to an item just from scratch. ( so you pay for any new bonuses just by themselves)
How do others deal with this?

[Java Man]

Read it again, final cost will be the same whether it is made in stages or all at once.

[Maezer]

There is no savings.

A +1 weapon is 2,000 + cost of weapon.
A +2 weapon is 8,000 + cost of weapon.

The cost to upgrade a +1 weapon to a +2 weapon is 6,000.
The cost to upgrade a +2 weapon to a +3 weapon (18,000-8,000) = 10,000
The cost to upgrade a +3 weapon to a +4 weapon is (32,000-18,000) = 14,000gp.

Etc. By the time you get to a +10 weapon, you've invested 200,000 gp into the weapon.

[ProsperoWilder]

The values are actually have that fir creation but this clarifies things greatly. Thanks a bunch.