Generally, spark plug tubes don't leak, but the round tube gaskets where they seal against the valve cover do. The valve cover gaskets are about $22 each, and the spark plug tube gaskets are around $3.60 each. So parts cost $44 ($22x2) and $28.80 (3.6x8). $72 + tax total, plus any other miscellaneous parts. Your dealer cost is practically all labor. So, if you have a trusted mechanic, I'd get the Lexus parts and have your mechanic do the work.
Instead of relying on a dealer, there are other sources to get OEM Lexus/Toyota parts as @RX400hmentioned.