I currently host many Invision Power Boards, so I've worked out alot of tweaks to make them perform optimal.
I've done quite a few servers, I normally do it by I tune the server, watch it for a few weeks. If you, other people, etc notice problems they get back to me I'll check into it. If no problems I stay outta your life till you have another problem, raise in traffic, etc.
The last forum/gallery I tuned had a average load of 35+ using apache 1.3 setup with a php cacher, its a forum that gets about 100users online, 80req/s. I got the load to never go higher then 1 point and some page loads (forum index for example, load in one second)
The prices I work at are very dynamic, depending on the site, etc. The normal range is 40-100usd. It greatly depends on your OS and how it runs, if you use cpanel the price also goes up due to cpanel being a server cancer takes alot of therapy to get it running good again
Please feel free to contact me @ cmanns91[at]gmail.com, give me a little details about your site, description of the problem, links so I may check it out myself. I'll give you a email back on if I can do much for your site or not
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