how long does the synchronization takes? I spent a week and it just reach 62% .. any offline files
Syncing takes less than 10 hours if you use SSD disks. However if you are using HDD disks (spinning mechanical disks) it can take many days or weeks, as the bottleneck is not the actual downloading but the validation of the blockchain, and that is is I/O-intensive. In your case, I would try a manual download of the blockchain (aka bootstrapping):
https://siasetup.info/consensusDoes Sia's excess storage compare to other existing storage? The storage price in Sia is determined from what? What system is used in Sia storage so that it can keep the stored files or data secure?
Sia works as a marketplace of storage, so the price is determined by each host setting up their own pricing. Hosts compete among them reducing their pricing to improve their score and so the chances of being selected for contracts. You can check the current marketplace stats here:
https://siastats.info/storage_pricingAs security measures, Sia uses the Twofish algorithm to encrypt all the data uploaded to hosts (only the renter is in possesion of the decryption key) and uses the Reed-Solomon data distribution algorithm to send redundant copies of the chunks of the files. The renter pays a 3x redundancy (already included in the pricing of the link above) and each file is chunked and sent to 30 different hosts: just any 10 of those 30 hosts are required to retrieve back the file (in other words, 2/3 of the hosts can go offline or lose the data, and yet the renter will have access to it)