The first one sounds slightly better - except you missed the 'r' in prices. 'now that' means something like 'now there is a situation where'. The second one is also OK.

"This is really convenient, especially now that fuel prices are so high." That is the correct sentence. "...Now the fuel prices are so high." implies that you have started a different sentence. Like this, "This is really convenient especially now. The fuel prices are so high. It is like you were talking to someone about how convenient something is, then you remark, "The fuel prices are so high."