Weapons in Rogue Trader-level campaigns are partially defined by their Availability - a stat that represents both price and ubiquity.
The list below gives a rough idea of the levels of availability, along with the appropriate TN for Profit Factor tests to acquire them.
Common (Very Easy) - Lasgun, several dozen lesser items (like ration packs, lho-sticks, etc).
Scarce (Moderate) - Demolition Charge
Rare (Difficult) Krak Grenade
Extremely Rare (Very Difficult) Digital Weapon
Near-Unique (Heroic) -Tempest Bolt Shells
The Scale of an Acquisition may affect how easy it is to make the purchase. The smaller the purchase, the better the bonus to the Profit Factor Test. On the other hand, purchasing enough power armour to protect a full regiment of soldiers would be beyond all but the most affluent of Rogue Traders - such large purchases will receive a penalty to the Acquisition roll.
Negligible (+10) - Single item
Trivial - (+6) - Squad (3–5)
Minor - (+3) - Platoon (10–30)
Standard - (+0) - Company (50–100)
Major (-3) Regiment (500–1,000)
Significant (-6) Division (2,000–5,000)
Vast (-10) Army (10,000+)
Of course, the GM is the final arbiter of what items may be purchased, and in what quantities - while purchasing 10,000 packs of lho-sticks to keep your personal army happy might not even need a roll, equipping each of them with Power Swords and their own personal Archaeotech sidearm will be beyond even the capabilities of the most powerful people in the Expanse, regardless of what the dice say.
By the same token, the area an Acquisition is made plays a large role in how likely it is to succeed. GMs should use their common sense - while a Forge World might be able to sell you an army's worth of weapons, a planet barely out of the Dark Ages and using black powder weapons won't be able to make that many of their own weapons, let alone the standard-issue laspistol.