A suitable standard constant current lamp would cost no more than £6-£8.00. The range in price tends to reflect the quality and the number of high output LEDs that make up the lamp and higher lumen output.
Your original question asked why there is a big difference in price between high and low cost lamps. Maybe I can offer some answers:
1. Any good quality constant-current control circuit chip together with associated components that are incorporated into a quality LED lamp costs considerably more than the selling price of the £1.00 lamp.
2. The less expensive lamps will not have any current control circuit other than a simple resistor. Its a lottery whether the resistor built in is sized for 12V, 13V or 14V, you wont actually know. This is quite important, if you apply 13.8V i.e., when your boat engine is running, to a lamp designed for 12V the current passing through the lamp will actually increase by around 50%. If you use a modern battery charger, increasingly these can apply up to 16V when running an equalising cycle, your cheap lamp will then be exposed to double its design current. In both instances the lamp will overheat and become rather unwell. At best you will have a very much reduced service life at worst a fire. Failure will occur rather unpredictably as the lamp light output deteriorates. A competitor actually shows an example of a burnt out product on his website.
3. High output 5050 SMD LEDs are mass produced. The lamps are sorted for both quality and colour and are binned accordingly into batches. These batches range from top quality ones, where light output and colour are closely matched. Down to LEDs that are outside quality standards, but still give an output. The best LED batches reserved for area lighting use, these are the most expensive ones and the ones used in quality lamps. The next lower quality LEDs often are used for OEM vehicle lighting where they are mostly installed behind lenses and don't really need higher quality attributes and are accordingly more attractively priced. The rest are often know as 'orphan' LEDs, they are unmatched in both colour and light output are often sold off cheaply. These are sold at about 5% of the price of the quality ones, often to small back room cottage industry assemblers.
4. The people who sell the cheap lamps will buy up these low cost assembled lamps for far less than £1.00. Clearly a profit needs to be made so one can only imagine that the assemblers component count involved in the lamp production is not going to be extensive. The quality of what components are actually used, following a similar theme, is going to be minimal too.
5. A reputable vendor will have a company registration and third party product liability insurance. They will also offer a full technical specification and a warranty for the lamps as fit for purpose. This is contrasted to the £1.00 seller who often offers none of these.
I don't think most boat owners appreciate that whilst cheap lamps look exactly the same and appear to work OK they won't really know what they have bought or indeed what is going on within the lamp. There is no way of knowing unless a full range of tests are made. I have found the electrical characteristics of cheap lamps can vary widely from the same batch dependant on what low cost components were used on the day of production.
Check out the reputation of vendors. This forum will certainly highlight any that fall short of the mark!!
Some food for thought!