Your services needs tuning to be interesting to people, I think.
Working for free is irrelevant to people when;
Basic services - "I'll take your salvage and leave your loot where it is, cutting into your possible profits and letting you collect the loot yourself, and I would not mind taking that too."
This only works if people don't really care about the loot/salvage. Essentially, if they are going to leave the wrecks ANYHOW, letting you have it is just a favor to you - they gain nothing from your 'services' beyond some 'feelgood' time knowing you got some isk that they did not care about anyhow. There is no practical reason why they should 'hire' you to do this at all.
Extra services - "I'll take you salvage and if you want the loot you can PAY ME to gather it for you. I get salvage and you get to pay for your own loot."
Dis-service. No-one are likely to agree to a 'deal' where you get salvage and isk for their mods - 20k for each wreck don't seem like much until you salvage a good level 4 field and haul around some 40+ wrecks or so, asking for 30-40 x 20,000 isk in all. It's like the lottery - you pay for a ticket and maybe what you get in the wreck is worth it, maybe not. That's 600k to 800k, for a decent-sized mission's loot. Whatever they pay you will be removed from whatever the mods were worth in isk or mineral worth.
Super services - you get salvage and isk for the loot WHILE the mission runner have to sit there and guard you while you cut into their possible profits. Nevermind that this may be an impossible task to begin with, but it kills their freedom of movement, something they might need.
In short, your services are not really services at all. Those who would like to hire your help are those who may want the salvage/loot, but have no way of getting it themselves or can't be bothered/don't have time to do it. Most mission-runners who have a second account have a salvager tag along to pick up the loot/salvage from the wrecks as they go, most who play solo need to take time to do both, salvaging after the mission is over. This takes more time, THIS is where you come in - performing the work of the salvage alt they don't have.
Basically this might be a better deal;
Appeal to those mission-runners that don't have time/ability/will to gather the loot and instead pushes for reward/bounty/standing from the agent and rats, and offer to salvage the loot for them for a cut, or a set prize, or any other means of making it mutually beneficial.
For instance, you could offer to do the job, and give them the salvage and possibly most mods. This will be a monetary gain for the missioner and a reason to let you tag along. Your pay would be any high-level meta item like meta 4 best-named and/or any faction items that may drop.
Or a cut in the salvage, or a hard isk bounty, or anything that you are both happy with. If I were in your shoes, striking deals with people who would forgo loot and salvage anyhow, I'd agree on what the missioner cares for the least - loot or salvage. Then I'd make my pay all of the items he/she don't want - for instance, I take all salvage, he/she gets the mods. Either of these are a good source of isk, high-grade mods can be sold, low-grade can be melted down to minerals - minerals are expensive to buy right now. Or you take the salvage and sell it - IIRC it's still a good source of isk because people always need salvage materials for rig-production.
Alternatively you build your own rigs and sell them on the market, building and selling whatever rigs sell the best, for instance. There are many ways, but the gist of it is to find a mutually beneficial deal. As things stand anyone what think over your offers for a while are likely going to accept ONLY if they really don't care for their loot/salvage and feel like being charitable to you, or enjoying the company while they mission. It's certainly not very beneficial to them from a financial point.