Runeforge Champion's last ability provides an alternative cost to cast Rune spells. You can't combine this with other alternative costs that may apply. In the unusual case that a Rune spell has additional costs, you can pay those in addition to the {1}. If those additional costs are mandatory, you must pay them in order to cast the spell.

Runeforge Champion's last ability doesn't change when you can cast a Rune spell.

Casting a Rune spell for an alternative cost doesn't change its mana cost or mana value.