I have had a few sets of boards made by JLCPCB recently and also find that prices appear to change in mysterious ways. I have only had small quantities made for home assembly.
Examples of “tweaks” worth trying include:
Change the board colour (“non green” colours take 2 days longer).
Switch between lead/tin and lead free, or vice versa, at least for home use being lead free isn’t an issue.
A set of 2 x 5 boards can sometimes work out cheaper than 1 x 10 (even allowing for postage).
Use as small a board size as practical, also let them cut boards to size, there is no advantage in having “snap out” board sets if home assembling (might be different for commercial assembly).
Reduce the number of vias if you are simply joining large areas on each side of a board to carry more current. 50 on a small board made no difference, 100+ did push up the cost a lot.
My guess, and it’s only a guess, is that small numbers of small boards can be arranged among batches of other boards, making for economies of scale. JLCPCB work with boards around 500mm square, then cut them out.
Trying various colour options is fairly quick, although unless you have endless patience changing the board size or deleting vias can be tedious.
Using the cheapest delivery to the UK, cost 3.85 GBP for 5 purple coloured boards 71 x 49mm, including PayPal surcharges, carriage and pre-paid 20% vat. That was a few days ago. Yet an earlier order for 5 green coloured boards 47 x 12mm cost 7.20 GBP! Bizarre isn’t it.
Expensive combinations to avoid are yellow boards with lead free pads, over ten times the cost of a green board (when checked a moment ago, it may change tomorrow).
For those wondering about shipping details, using their cheapest option seems to involve sending a batch of boxes via air freight from Hong Kong to London, then posting them via Royal Mail 48 hour tracked delivery. From uploading the zip file to the postie handing me the package takes around 10 to 12 days.
SJ