Hey! Hope it's alright for me to jump right in!
You'll have to generally beat all the tiers for the Prize Fights of the one you're aiming for (meaning, for Gold, you're looking at past 1,000,000 points!) to receive a gold fighter at the very least!
If you really wanna aim for the gold, I wanna say max out your silver fighters first and from there focus on gold! Otherwise, don't be afraid to grind anyway on gold so that way, even if you miss out on an actual gold character, you'll likely get a Silver character advertised there. It's rather tough later in, for the Gold tier, but you can totally get through with a little elbow grease and some Blockbusters!
For example, if I'm correct and Filia's prize fight event is still on, beat about half of her event and you'll get another Hair Apparent, but go all the way and more, and you'll receive a Parasite Weave, and some gold moves!
And on terms of theonite saving, you'll have to keep grinding the daily missions they have every day until the newest update which will give us Daily Logins! Beat the story modes for now and save up big, I'd say! Your level up rewards also help, but, sooner or later it will run dry so, don't rely on it much!
So, don't give up and keep at it, dude! You'll be up in the big leagues in no time!