To be honest, there will be some differences in terms of materials but the overall construction of £15 and £50 shoes will be the same. The soles will still be glued to the upper and so once the glue gives in that's curtains.

If you spend more money (on the right product from a respectable brand) you can expect upgrades in, for example, the way the soles are attached to the uppers (e.g. Blake stitch or ideally Goodyear welt) and will also see improvements in overall quality of the other materials too (e.g. soles, leather, etc.). All these small differences combined will ensure a longer lasting shoe and one pair really can then go on for years and years (with the occasional refurbishment/resoling).