Seen a book for sale, in a catalogue, a shop or on a forum?
Before buying it is always worth using the power of the internet to shop around – prices vary dramatically. Of course the condition of a book is a major factor in the price, and your own collecting criteria (perfect condition only, slight imperfections or can tolerate “reading copies”?) will guide you to the “right price” for you. Even if you find the book on-line, you can always do what my father did and call the dealer, have a chat and usually agree free posting or sometimes a considerable discount.
Beware of facsimile reprints or POD (print on demand), these are cheap but in ones I have seen the binding (known as the oxymoronic “perfect binding”) doesn’t last long and more importantly fold out maps are not reproduced but simply copied folded up, i.e. useless!
If you are after a reading copy then a digital copy may suit, so search the internet for digital copies, many are out there. Electronic copies are offered in a variety formats: pdf, kindle, daisy, html, jpg. If you don’t have a Kindle then you can download Kindle Reader for free from Amazon to run on your pc/tablet. Forgotten Books also offers digital copies for a small subscription. Many of these you can get for free elsewhere, but the quality from the website can be better. You will probably not get the maps as in POD but in an electronic copy the binding doesn't break and it may be free.
To take a recent example:
With the Scottish Yeomanry: Being a Reprint, Somewhat Altered & Extended, of Letters Written from South Africa During the War of 1899-1901, TF Dewar MD, Arbroath: T. Buncle & Co. 1901
Offered recently by a specialist dealer for £75 in good condition with front ffep (front free endpaper) missing – a small imperfection. For a book about a small unit this seems like a reasonable price, a quick check of Bookfinder.com and abebooks.com shows this to be the case, with a number copies to choose from £113 to £145. However, there was one copy for £48 in good condition, some colour loss on spine and top half of the front endpaper cut off. So, very comparable to the £75 copy but £27 cheaper, both are in the UK so postage is not an issue.
And of course before buying, just double check you haven't already got a copy. There's a good quality copy of With the Scottish Yeomanry for sale.