I've found out why there is such a big price difference between pens. The cheaper ones have steel nibs (which may be gold plated), and steel nibs are stiffer. At about £100 you start to find solid gold nibs, which are more flexible. Beyond that, it's finish and brand name.
A rather simplistic view and one that is not necessarily true I'm afraid. For example, the steel nibs fitted as standard to Pelikan M200 (and similar) pens are more flexible than the gold ones on the more expensive, but identical in size, Pelikan M400.
Despite what may be stated by manufacturers, there is no clear advantage to having a gold nib over a steel one. However, customers expect a more expensive pen to have a gold nib, hence gold nibs are often finished to a higher standard. It's a bit like comparing a rear mech made of CF to one made of steel. A steel one can shift just as well as a carbon one
all other things being equal. However, all things are not necessarily equal.
One online pen shop has a good guide to this sort of thing, and concludes by saying that £300 is about the limit beyond which you don't get a better pen, although you might get a "nicer" one. Same as bikes, guitars and other personal accessories, then; above a certain point you get seriously diminishing returns.
I'm afraid I don't agree with that either. £300 is quite a hefty tag and it is not necessary to spend anywhere near this amount to own a well-performing fountain pen that is a joy to use. The aforementioned Pelikan M200, at typically around £50, can write just as well as pens costing many times that figure. The issue is that, sadly, almost all pen vendors just can't be bothered, or simply don't know how, to set up a pen to write at its best. There was a time when pen shops employed full time repair technicians and he or she would ensure that a pen writes perfectly before it leaves the shop. These days, pen shops are staffed by untrained personnel in the same way as Halfords is not a proper bike shop. Thankfully, there are one or two exceptions.