How to use the chart (so far, weapons-only...):

Step 1: Find a Power for your weapon. From this, you will find the corresponding Rank, the number of Add-on slots it has to put cool stuff into it, and the Base Price. You may also see some basic restrictions which are to be followed.

Step 2: If you have a sub-S rank weapon, you may choose to increase it's Rank, giving you more Add-on slots, but increasing the Base Price.

Step 3: You may now choose to put restrictions on your Weapon, which also increases the Base Price, and allows you to either get more Add-on slots, or more Power. More restrictions, here, too... Can't give a 120/120 requirement to an E ranked weapon and gain tonnes of Add-ons... you'd have to upgrade it to S, first, which increases the cost ^.^

Step 4: The fun part, you pick the Add-ons you want to put on your weapon, taking note of their Add-on Ranks [1-10], because restrictions get heavier here. They state how many Add-on slots they take. They also have a multiplier, which you multiply your Base Price by to get the current price.

So, for example:
Bob makes a cool sword. He likes his sword, and wants to make it cool, but strong enough to be useful, too.

Bob picks a 41-45 Power Range. This makes it B rank, gives him 30 Add-on slots, and costs 1,200 Base.

He then upgrades it to an S rank weapon. B to S is a +10 Add-on bonus, and increases price by 4,200. The Base Price is now 5,400, and he has 40 Add-on slots with no restrictions as of yet.

Bob is a rather Strong guy, with 90 Tools and 85 Strength, so he plans to add restrictions to his Weapon in order to make it cooler. Instead of taking the 100/100 ones, which he'd have to train to use, he'll use the 80 Tools/80 Strength restrictions. This increases the Base Price by 650, and he wants +9 Power. It also meets the B+ rank requirements for this.

So, after all the prep-work is done, Bob has:
A sword, S-ranked, with 40 Add-on slots, 50-54 Power, with a Base Price of 6,050.

Now, to make up for the lower-end damage, Bob gives it a Bleeding Effect. He picks the 15 Stamina for 5 posts, costing 25 of his slots, and preventing him from getting another effect of Add-on Rank 9 or 10. Bob is fine with this. The cost multiplier is x2.8.

Bob also wants some +Stats on his sword. So, he chooses a +10-11 Stat bonus, picking Strength as his bonus [for more Melee Tools damage], costing 14 Add-on slots, fitting well within his restrictions, and giving a x2.5 multiplier.

So, at the end of the day, Bob has:
Custom Sword
S-Ranked
Must have 80 Tools and 80 Strength to wield (+9 Power chosen)
50-54 Power
Bleeding Effect: 15 Stamina per post for 5 posts
+Strength = +10-11.
Price = (6,050) * (2.8) * (2.5) = 42,350 Yen

Ware sees this, and gets out his AIM chat, and rolls.

He rolls a 1d5 + 49 for the Power. Let's say he rolled a 4: The person would have a 53 Power sword.

He'd then roll a 1d2 + 9 for the Strength, and he gets a 2. So, +11 Strength.

So, for 42k Yen, the player gets a really, really nice, S-ranked custom sword, which deals 75 damage from bleeding [split over 5 posts], and gives an impressive Strength bonus [note: This bonus cannot be used to meet the wielding-requirements... if you can't wield it, you can't get the bonus to begin with ^.^].

It's expensive, yes... but once people start getting Mission cash, it'll be easier to achieve.