Some critiques from me:
Not bad for a first design, nice to see someone not using tables!
I would say the site design needs to be larger/wider, typically 960px wide is considered a good size for a site
I'm not digging the logo for some reason. it just doesn't seem to work very well to me
I think you need to be consistent with your font colors, for example some of your text is white and some black, but it would probably looks better if it was all one colour (i'd say white > black on that site colour scheme)
I think you need to fix the site nav bar a bit, on some pages you see different items which doesn't make much sense! if your navbar is static like that you may want to look at using Includes to attach the same nav bar to every page (ASP/PHP includes are fairly straight forward)
You have a contact section on the homepage and a link on the navbar also to a contact us section, my advice would be to ditch the contact section on the index page and just have the contact page from the navbar, it may be worth while to add the phone number somewhere on the homepage for the lazy among us though
I think the navbar text/buttons need to be a bit larger, say 15-20% bigger
If you click 'services' then there is a gap at the bottom between the content div and the footer (this is in IE 8.0 with compatibility mode turned off, with this on it's OK)
The page is not "centered" properly for me, it's edging towards the left more. Best way to resolve this if you want a centered layout is to create a div for the page as a 'wrapper' and give it a fixed width, in the CSS give it the value margin: 0 auto; to centre it
i.e in HTML:
<div id="wrapper">Content Here</div>
in the CSS:
#wrapper {margin: 0 auto;}
As for how much effort you want to put into it the question is how much you get paid, as you've said you are looking at getting £100 it's probably not worth spending an endless ammount of time on this unless you want to create sites for cash generally, in which case any experience is good experience and i'm sure a good portfolio will speak volumes to potential customers, who will want to see your work!