how much gold would I need to spend through the process of completing skyscale achievements?
Refuge from the State of Dota
It depends on whether you craft some of the items yourself or buy them. If you don't have charged quartz, start charging now. You need 22 and can only charge one a day. 12 for food and 10 for a heat lamp.
Bare minimum is 41g.
You have to spend at least 41g - 6g for toys during Raising Skyscales (part 3) and 35g for saddle components during Riding Skyscales (part 5). Other costs depend on what you're willing and able to craft and how many jumping puzzles, mini dungeons and world bosses you're willing to do.
You'll need 12 pieces of Skyscale food for Raising Skyscales which cost just under 2g of materials to craft or 2.6g - 3.2g to buy from the TP. You'll also need a grow lamp which costs 12g to craft or 19-22g to buy. The biggest barrier to crafting both yourself is that they need charged quartz crystals which can only be created once per day, but if you're not in a hurry that can save you quite a bit, especially on the grow lamp.
Then during the Skyscale Lost collection you have the option to use an extra-pungent skyscale treat to skip each step. You get one treat for free and can craft more at a cost of 4.6g for materials (including more charged quartz), or buy them for 5-6.6g each. There's 21 objectives total in that collection.
So the minimum is 41g and 36g worth of materials (which you might already have), and you could earn back 6.6g if you don't use your free skyscale treat and instead sell it (listing it rather than sell instantly) for a total cost of 70.4g. The maximum - if you skip all steps of Skyscale Lost and buy everything instantly from the TP instead of placing buy orders or crafting is 233.4g
(Bear in mind all these prices - except the 41g at the beginning - are based on TP costs and therefore subject to change.)
My advice if you want to keep the cost down is to check how much charged quartz you have (it comes from Wintersday gifts among other places so you might be surprised) and if you have less than 32 start charging it up once per day now. Also check what other materials are needed for the grow lamp, food and toys and start saving them but don't actually craft anything until you need it (the crafting isn't time gated, except the grow lamp and you only need 1 of those anyway). Then when you get to doing Skyscale Lost try doing each step yourself and/or by asking your guilds or in LFG and only use a treat if you're really stuck. You'll probably find there's at least some you can get done easily without having to use a treat.
Finally, if you're concerned about being able to earn enough gold you should also bear in mind that the saddle costs 250 of each Season 4 map currency, 1,500 volatile magic and 500 trade contracts so you might want to start saving those up now. (And you can do things which will earn gold like map meta events or gathering materials to sell at the same time.)
Danielle Aurorel, Desolation EU. Mini Collector
"You can run like a river, Till you end up in the sea
And you run till night is black, And keep on going in your dreams
And you know all the long while, It's the journey that you seek
It's the miles of moving forward, With the wind beneath your wings"