High Guard (performance section?) and CSC have some rules that may be applicable.
I don't use book descriptions nor prices - my players get custom lists - and I don't use fixed prices. I factor in supply and demand aspects. I don't run 'trade' games - I run adventures.
Its more work, but has worked really well for me. For what its worth, here are some guidelines I use:
Higher TL goods on lower TL worlds
- non-essential/luxury are more, unless common [Dyson vac; 3D TV vs. standard vac and TV]
- essential (ala breathing masks) may be dirt cheap
- military/para-military items may have an even higher premium
- repair is expensive or unavailable for non-essential/luxury
- greater distance to nearest source increases price
Lower TL on Higher TL worlds
- Generally cheaper, unless an 'antique'
- repair may be costly/unavailable
Law level, government type, local customs and current events may cause prices to vary even more.
I inform my players that, just as in RL,
all prices are negotiable (yes - I've even negotiated prices at Walmart, Lowes, etc. as well as for apartments, appliances and parts - and even some online automated systems often have built in 'negotiation' ala Dell). Though, just like in RL, one has to factor in their own time and effort in determining what is worth it to negotiate. (Represented in game by reductions of 1 Cr and the like - and increases if they get silly...)
Of course, attempting to negotiate with a clerk may often be futile. Likewise NPCs that 'dislike' the PCs for any reason (cultural or particular). PCs must have skills or stats that support this - or be really good at roleplay (though advantages are limited). Difficulty DMs are the key to making this work.
I also let the players know they may ask for things that aren't listed - and I make them up (or not) as I see fit.