I'm travelling from Newcastle! Just checked the price to Haywards Heath only 60p cheaper. It is really slow though, nearly as long St Pancras to Brighton as Leeds to Kings Cross
Cheapest is via St Pancras £11.80 return. It is pretty slow though and includes a change at Three bridges. Only need a ticket to Haywards Heath as your match ticket includes travel from there (though I forgot that so you might be able to get it cheaper than £11.80). As you are coming from Leeds...