The short answer to the question you actually asked is this: when you upgrade a Legendary, you have to buy the new base piece, but can re-use the Soul Ash and Soul Cinders from the old one.
So: if you say make a 190 legendary, and later want to upgrade it to a 291 legendary, you won’t lose any of the Soul Ash in the process, but will have to buy a 291 base piece, so whatever gold you paid for the 190 will be ultimately wasted.
A lot of legendaries aren’t worth much, in practice. Each class has only two or three legendary powers that are much worth having, and of course, you have to earn the legendary Memory for each to embody it, so rushing to make a legendary first thing is usually not going to be a good idea. Take your time, get the right legendary, and wait for a good deal.
You’re not 60 yet, on that character at least. Or maybe you have another character that is 60.
I would say that the best way to progress gear towards a legendary for a new 60 at this stage of the expansion, where you don’t have a 60 already, is:
Go to the AH and buy Greens of ilevel 229. They are plentiful and should be relatively cheap - a few hundrred gold each. You may run into supply shortages and so higher prices, on necks and maybe rings, but get what you can.
Open Zereth Mortis and do the campaign. It doesn’t take too long; a day or two, depending how long you play. That will get you the Unity L:egendary Belt of ilevel 265, which may or may not do a lot for you, depending on your Covenant. See https://www.icy-veins.com/wow/legendary-powers-guide for details about that.
Do Stuff in Zereith Mortis, not only to improve your Cyphers, and hence gear, but also to accumulate 3000 Cosmic Flux to buy a Bag of Explored Souls, which has enough Soul Ash and Soul Cinders to create a Legendary. https://www.wowhead.com/item=191701/bag-of-explored-souls This will save you burning out running Torghast.
When you have consulted icy-veins.com about the best Legendaries for your class, and you have obtained the Memory you want, now buy the base piece you want. Remember that each Memory (except the free Unity) can be inscribed only on two specific slots, so you have to buy a base piece for one of those two slots. Prices of base pieces are weird. For example, on your realm, I see a 190 belt being sold under 1 gold right now! https://theunderminejournal.com/#eu/pozzo-delleternità/search/grim-veiled whereas a 291 base piece belt goes for 75,000.
I was overlooking how expensive base items are, so crafting a level 7 legendary is definitely the way to go to save money.
Just one quick question, cheapest way to get a level 7 base item is to upgrade a level 4, isn’t it?
Sorry, I’m not sure what exactly you’re asking, and I’m pretty sure I wouldn’t know the answer if I did, since I haven’t got into crafting Legendaries this expansion.