Approximate calculations using 100 as TDS for ease of use.
Resin only: 500,000/100 = 5,000. That's how many litres of pure you ought to get per 25 litre bag of resin (£75??). Resin seems to be more variable of late though. Add to that the one-off cost of two reasonable-sized resin vessels. Start cost =2 resin vessels. Maintenance cost = £75 per 5,000 litres of pure.
Using RO: Could be anything really depending on amount of water required and whether water pressure is sufficient to avoid using a pump. Start cost: Housings, RO, pre-filters, possible pump and one resin vessel (not always needed at first but better to make sure as will probably be needed later as RO gets older). Maintenance cost: Changing RO membrane(s) every few years, changing pre-filters 3 times a year (if 20" - more if 10") - pre-filters might be £25 a go, very occasional resin change.
Either way, a submersible pump and transfer hose should be needed.
As a guess, say £75 a year for pre-filters (depends where you get them) and an average of £40 a year for membranes plus maybe £40 a year for resin so £155 a year - but for resin only, depoending on production needed, £1,800 a year (2 bags per month).
More can go wrong with an RO (though usually easy to fix) but regularly filling resin vessels can be a hassle.
There is a lot of variation possible depending on your requirements, so don't rely too much on the above.