My own (admittedly limited) experience of PCP deals makes me think that the option to buy price is deliberately set at a low price to encourage you to buy the car at the end rather than just return it. I bought a Polo GTI with an option price of £8,300 when the market value was nearer £11,000. I therefore had to buy the car and sell it myself so I wouldn't lose the £2,700 difference.
This 'profit' should therefore be factored into your decision along with your deposit and monthly payments.
either that or;
it's set deliberately low, so then you have some "equity" in the car and the dealer gets you to use the difference as the "deposit" on your next PCP, thereby keeping you in