It depends on the contract, but there are varying rates based on "qualifications". Negotiating, per se, is not part of the package, but it may be done by "reclassifying" a tech.
Example: I worked at Beaver Valley in 2001. There was a chart of pay rates that looked like a tax table. I was a Sr. HP, NRRPT, non-returnee. I was paid $19.76/hr.
Two years later at SONGS, I was paid $21.00 for being a non-returnee with >7yrs as a Sr. HP. The next year, I got $22.50.
I put "qualifications" in quotes, because they use arbitrary things like the >7yrs or being a returnee. Even the term "returnee" can mean many things. sometimes it means that you worked that site within two outage cycles; other times it means the you worked in that system.
If they really want you to go to an outage, but the money isn't enough, they can sometimes get you in as a lead tech. or as an ALARA tech. ALARA is the big cookie of the HP business. It is the way they can pay an HP tech (with no more education or qualifications than any other ANSI 3.1 tech) an extra $5 to $10 an hour by giving him a desk and making him hang lead for the outage. There are only so many of those slots, and they tend to go to the most capable techs, but sometimes they go to "favorites".