Lets say you have just started war dragons and want to buy a mythic from nothing. So how many architects packs does it take to get the mythic divine? An architect pack contains 15 gold chests and 35k rubies. I opened super sigils with 180k rubies and that yields 44k sigils. I am taking this as a reference.
Since you want a rider too, venus is good for you, 37k sigils needed and 6 keys collected. Next cheapest line is omniros, 35900 sigils needed, 6 keys comes from this line. 3th line can be electrum line which is 37k sigils and 6 keys. Next cheapest key cames from cosmic line, 4700 sigils, 1 key. Last key can come from mission boost line, 5850 sigils, and the last key obtained.
In total, 120450 sigils needed. If you only use rubies, not any gold chests obtained from lines, because you are rich, that is 490k rubies.
you can buy a mythic with 14 architect packs without opening gold chests obtained from that lines. In my country it costs 691 dollars