Chargeable gains – Chpt 13 example 5

Class notes example 5 pg 121

Please tell what is the base cost on the 750,000:

New purchase cost – 750,000
Less ROR (70,000)
Base cost 680,000

If alternatively we use 800,000 for the cost, why because in previous examples she used the new purchase cost and not the original purchase cost?

Thanks for taking the time to answer my question.