Compare the full legendary runeset production cost to just buying all the runes you can instead (both cases full sets).
After hopping to wiki and doing some data crunching it turns out the total cost of buying all regular runesets on tp is around 923.83 gold. Do keep in mind some runes are only craftable or obtainable via dungeon vendors, so there's that on top of the TP price.
Cost of crafting a full legendary rune set hovers around 2,596.35 gold at this moment. Also there seems to be total of 83 different runesets, so take that to account too (bag space).
What's the conclusion? The conclusion is that you make your own conclusion! I just felt like providing myself, (and now you along the way) this data, and with it a rough idea how the legendary vs standard rune costs look like;)