Bidding for an upgrade via MMB

So I have just booked a flight to Boston for this week. As travel funds are all gone due to end of year I have had to book Econ :0 Still lounge and the red wine will take a big hit before hand
As I look at my booking I get the normal options to upgrade to PE both ways for a cost of £300, not that bad TBH. Underneath this I also have (for the first time ever) the option to "Bid For an Upgrade". This is great but, you knew there would be one, the lowest possible "bid" is higher than the cost to upgrade normally.
Is this correct? Do I make a call to CS to say I want to bid lower than the sliders allow as it higher than the cost to upgrade?? Any knowledge to share would be great.
Thanks


As I look at my booking I get the normal options to upgrade to PE both ways for a cost of £300, not that bad TBH. Underneath this I also have (for the first time ever) the option to "Bid For an Upgrade". This is great but, you knew there would be one, the lowest possible "bid" is higher than the cost to upgrade normally.
Is this correct? Do I make a call to CS to say I want to bid lower than the sliders allow as it higher than the cost to upgrade?? Any knowledge to share would be great.
Thanks