Open DATA\EQUIPMENT\Market_commodities.ini. Decode it with bini of not done yet. You should see lines like these:

[BaseGood ; <= bracket
base = Br01_01_base
MarketGood = commodity_cardamine, 0, -1, 0, 0, 1, 0.880000

The last number is a multiplier for the price.

If you open the file DATA\EQUIPMENT\Goods.ini, you will find things like this:
[Good ; <= bracket
nickname = commodity_sidearms
msg_id_prefix = gcs_gen_commodity_sidearms
equipment = commodity_sidearms
category = commodity
price = 280 ; <================================== the default price
combinable = true
good_sell_price = 2.550000
bad_buy_price = 2.550000
bad_sell_price = 1
good_buy_price = 1
shop_archetype = Equipment\models\commodities\nn_icons\cwire_munitions_1.3db
item_icon = Equipment\models\commodities\nn_icons\COMMOD_lightarms.3db
jump_dist = 4

It shows the default price for that commoditie. This is the price that will be multiplied with the number used in market_commodities.ini



In your example, search for Li01_01_base in market_commodities.ini. This is planet Manhattan. Next, find this line, or add it of there is no line like this:
MarketGood = commodity_cardamine, 0, -1, 0, 0, 1, 5
and change the last number to whatever you want, like 15.

!!!!! Remember, the last number is a float, so in case you use a decimal notation, ensure you have exactly 6 numbers after the dot.
this is a correct notation of a float:
1.100000
this is a wrong notation of a float:
1.1

What's bini?

An tool to compress and decompress ini files.
You can find it here:
BINI Toolkit

If it is to complicated, I recommend to use Freelancer SDK instead. VERY easy in use. Simply execute this tool once, and it decompress ALL ini-files in the directory you choosed.
This is the url: Freelancer SDK 1.5.

The SDK is actually better, the Bini sometimes mess up the files, the SDK is all decoded for you.