Get ready for a loooong post
It all depends on how long it will take you and the competition on keywords.
You say not much
SEO was taken into account when the site was designed? In that case you may have to slightly alter all of the pages listed. If the site is html then it is pretty straight forward, add <title> <description> tags, <h1> <h2> and all of the usual ballony. <alt> tags in images, keywords in inbound anchor text links, etc, etc.
Weigh up the competition. A search for a local caravan park should not really be very hard to rank for as compared to the mobile phone and finance industries.
I suppose the best way would be to give him a detailed report of what needs doing. If you need to alter the pages, offer him so much per page to make them
SEO friendly. Then you need to do a backlink campaign, again add that to the price. Directory submission, forum links, press releases, etc. You need to see how much work is involved and break each and every detail down to a price that you deem fair.
A basic
SEO campaign could go something like as follows:
Keyword research for say 10 pages - time estimated.
Alterations of each of the 10 pages optimised with the chosen keywords from your research - time estimated.
Backlink campaign, maybe offer so much per link.
Directory submission, cost per so many submissions.
Blog comments on related blogs - time estimated or cost per comment.
Writing articles - cost per 300-350 words.
Press release - cost per 500-600 words or more.
Press release featured on Google, Yahoo, MSN, Ask news sites, additional bonus.
Social Bookmarks - Cost per 50 submissions.
Most of these should help his site to rank pretty decent for a keyword with hardly any competition. Putting a price to
SEO is not something that you can give at just one price, as it cannot be estimated just how much time it can take. If he trusts your judgement in
SEO then he should let you do it ongoing, as you should never stop promoting your site(s).
You should break down your plans and show him what you intend to do and at what price per sector. This should help you to reach an overall price.
If you can find people who you can pay to do services for you, then by all means go ahead and do it, act as a middle-man and add a little to each price. After all, he is paying for your wisdom too.
The following I would do if I had to do an
SEO campaign. The prices are guesstimates as rates change, and is using the services of reliable people I know (coz I'm lazy

)
Keyword research for say 10 pages - time estimated 2 hours (estimate). Charge at hourly rate.
Alterations of each of the 10 pages optimised with the chosen keywords from your research - time estimated 4 hours (estimate). Charge at hourly rate.
Backlink campaign, found somebody who will get me 25 quality backlinks from other similar themed sites for £30, I charge £40.
Directory submission, had a quote of £25 to 1000 diretories. I charge £35.
Blog comments on related blogs - Easy to do yourself on dofollow blogs. Maybe charge £1 per comment.
Writing articles - 300 words of pure original content, quoted £5 per article. I charge £10 per article.
Press release - 500-600 words plus submission to 5 news sites. Quoted £7 per article, charge £13 per article.
Press release featured on Google, Yahoo, MSN, Ask news sites, I add as an additional bonus. Would be nice for a bonus though as sometimes it can bring a site 1000+ visitors per article + backlinks.
Social Bookmarks - Quoted £10 for submission to 50 dofollow bookmarking sites. I charge £20.
Article submission service. Quoted £25 for 500 article directories. I charge £35.
So for instance if I was charging £10 per hour for the keyword research and alterations to the webpages a total outcome would be £50 + profit on all campaigns for 5 hours
work.
Then additional...
1 Backlink campaign profit = £10
1 Directory submission campaign profit = £10
20 blog comments = £20
1 article profit = £5
1 press release profit = £6
1 Social Bookmark campaign profit = £10
1 article submission service profit = £10
Total profit = £121.00
Cost of campaign to customer = £223.00 (which is a very fair price seeing as some companies charge in the region of £1k +). Break down the timescale it should take, some people take 24 hour turnaround to write an article/press release that is quality.
You could add on extra for time if you decide to do the campaigns yourself, but is very time consuming and boring. Or add on extra if you think the package you have offered him is worth more, I don't know. It all depends on the quality of the visitors you are sending to him.
It wont be worth trying to optimise his site around his company name, as that wont be worth much to him as the visitor already intends to stop at his caravan park, it would be new business he is looking for and its up to you to deliver. Plus if his domain is generic then he should rank automatically after any
SEO campaign.
If it is in essex for example, you need to target keyphrase like 'caravan site essex' and similar.
Anyway, I hope that helps

, And good luck!
Terry