You'll end up paying either 45,000 gp or 45,125 gp
(depending on your interpretation of the multiplications rules in PF)
First of all, as our mystery guest noted, a character can't normally benefit from two traits from the same group (magic traits in this case). A DM can always overrule that, however, and in that case, two things to note:
First, the discount is from the creation cost - not market price (this will come into play later):
Benefit: Whenever you craft a magic item, you reduce the required gp cost to make the item by 5%.
Spark of Creation
Benefit(s): You gain a +1 trait bonus on Craft checks, and the cost of creating magic items is reduced by 5%.
(Emphasis mine in both)
This means that for an item with market price of 100,000gp, the discount for each trait separately will be only 2,500gp (5% of 50,000) and not 5,000gp.
Second, I'm not sure it applies here since this is not a roll but a static number, but note that multiplying in D&D works differently than normal algebraic multiplication.
Taken from PF SRD Common Terms:
Multiplying: When you are asked to apply more than one multiplier to a roll, the multipliers are not multiplied by one another. Instead, you combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiple. For example, if you are asked to apply a ×2 multiplier twice, the result would be ×3, not ×4.
This is a somewhat awkward way of saying that the multipliers are applied to the base and their effects are added (so X2 is really just "base + 1 more" and X3 is "base + 2 more", combined they yield "base + 3 more" or X4, not X6).
So, two options:
- If this does apply to your case, than 5%+5% are 10%, so the 50,000gp creation cost of the item will be reduced to 45,000gp
- If this doesn't apply, and you calculate the multiplication the same way a shop-keeper will do for multiple discounts, you'll get
0.95 x 0.95 x 50,000 = 45,125 so the item will cost 45,125gp.
Note, that if you consider the multiplication rules to apply to your case, and ignore the first point. than you'd add all the multipliers for
0.05 + 0.05 +0.5 = 0.6, and end up with paying only 40,000gp as you suggested.