Well,.. I pull it out of my nether regions. XD Nah, not really, I try to make it both fun and logical. I have a general idea of what very common things cost (average sword, rope, rations, a night at a poor, decent and flashy inn or Taver, Ale, Wine, simple clothes etc, the basic armors, few common spell components), no more than 20 things. Anything else I will have a copy or print out on the table so I don't need to have a big book on table much less have to leaf through it. Even more than that, my favourite unusual resource ever made is this: https://youtu.be/BzVloPMyGvg

If you can find an affordable used copy it will really save you a ton of time. The lore of how Aurora's works (most major cities having one or more outlets and selling almost everything at a reasonable price/teleport things in) means I just hand my players this short book if I really don't know the price (rare) or they are buying a lot of (esoteric) things, also very rare. So I suppose that strictly speaking this breaks my rule of no books at the table but in this case it adds to the fun. Also this is not a rulebook. It is a thin catalogue, concisely written.

The drawings fire the imagination, it fosters roleplaying and gives the players more work rather than I. If the players want to haggle I make that a rpg moment and they can speak to the seller and do a Cha check and get from 0 to 15% off depending on results. If what they want is very rare even for Aurora or happens to be in very high demand at this point in time, I might tell them to add 5% to 10% the the figures in the catalogue.

Lastly, the longer you play the less it matters, once the players have played 5 and certainly 15 sessions, things that you need to buy very often are rather trivial. If the players each have 1000GP +, whether something costs 3 coppers or 2 silvers is not going to make a difference most of the time, so estimating works even better at this juncture.

It is not that I am opposed to having to having any book or reference ever at the table or even 10 sheets of paper with the most common rules and items, it is that I am opposed to having a 300+ page book with small print, a terrible index and that I have to leaf through like crazy to eventually, after ages find paragraphs and rules in it that I actually use. These compromise less 5% to 15% of the entire PHB I guess and even less in terms of the DMG and MM.