It's been 8 years since the new pension, each year costs around £1,000, adding £6/week for each year you pay, so for £8,000 you can buy an extra £ 50 pounds a week, or £2,000 pounds a year, getting you 8k back in 4 (ish) years. Or viewing it as an investment, a return of 20% a year.
Obviously adding extra years won't get you more than the maximum new pension.