1/ with BTC - is 0.005 USD per hash (my guess) expensive ? How much is it with FCT ?
See
https://shop.factom.com/You get 1000 Entry Credits for $10 = $0.01 per EC. One EC can hold up to 1kb of data. A SHA256 hash has a size of 32 bytes. So ~30 hashes fit into one entry.
30 hashes on the Bitcoin blockchain would cost 15x as much = $0.15 (when I use your numbers).
However, transaction fees vary on bitcoin and tend to raise over time. You also don't want to bloat the bitcoin blockchain with billions of hashes.
2/ prices may vary, but for both I'd say
Yes, probably for both. Not sure on Factom. However, Factoms masterstroke is, that this doesn't affect the price if you want to store data. The price is fixed at $0.01 per EC. In the moment someone buys EC, a federated server calculates how much factoids are worth $0.01 and burns them. If a Factoid is worth $0.01, 1 Factoid is burned. Is it worth $1 than only 0.01 Factoids get burned. The customer who wants to store data doesn't have to care about this.
3/ custom blockchains probably not with BTC, but there are other solutions, and what is the reason to try with FCT if the data is not guaranteed to be stored ?
I guess I don't understand what you mean.
4/ ?? you "buy" xxx ?? What is the difference ?
From step 2: when someone buys EC (which are needed to store data), then he would need Factoids which get burned for these ECs. However, Factom Inc. handles this for its clients. They just say "give me 1k EC please". Factom says "ok, here it is" while they are automatically creating a wallet, sending factoids to it and convert them into ECs. Since
EC are not tradeable but just a token to store data and because it's
value is fixed at $0.01, the customer technically doesn't own cryptocurrency and never needs to have a complicated bookkeeping to record the current exchange rate at the moment the data is stored. Not to mention the regulations involved when you buy/own/use a real cryptocurrency.