Depends on what the route is as for the applicable fare

A Y basis ticket can be very expensive , would expect it to be more expensive than discount PE and possibly more than discount business

What is the route?

As an example, JFK-LHR r/t starts at $591 r/t plus taxes in O class whilst Y class starts at $4718 plus taxes/charges
Premium economy starts at $1081plus taxes/ and business at $2216 plus taxes

If upgrading using miles, then it is likely better value to use the higher mileage plus co-pay that is needed for O class, than to pay for Y

e.g. for JFK-LHR it would be 25,000 mlles plus $350 from discount economy each way vs the 15,000 miles needed to upgrade from full economy

the extra 20,000 plus $700 may well work out better value

Note: you would have to pay any additional taxes too regardless of which upgrade you use