EOS tokens on exchange services costs around $18 (e.g. eoscountdown.com), but at eosscan.io, tokens for each sale period starts from less than 1USDT. Is it the same token? Otherwise, what's the difference between one and another?
EOS tokens traded on exchanges are tokens that were purchased from the EOS ICO. The price of a token when buying from the ICO is determined at the end of the 24 hour period, meaning you can expect the price to climb through the day. The value of the ICO tokens for that 24 hour period is relative to the value at which speculators are trading the tokens on exchanges.
In short, it's unlikely that you will find the ICO token price very different from that what is available on exchanges for that particular 24 hour period.
It's the same token. It's likely throttled throughout the day. The most accurate depiction of cost is closer to the end of the sale period. No matter when you submit your ether during the sale period it will always go with the rate at the end of the period. The rate is based on demand during the specific sale period, but will likely be close to the price on exchanges (maybe a little more or less).