It really depends on whether or not you like paperwork and if there are any issues with your application (medical problems, custody issues, criminal records, sketchy work experience, approaching 45, etc).
I did my application all by myself. My husband is an engineer and getting the skills assessment was very simple (copy of resume and degree) so there wasn't any complications there. We knew our points and work experience was ok and that the medicals and police checks would be fine. So for us it was fairly straight forward to do it ourselves. At the time though I wasn't working and I happen to like researching and paperwork so I didn't mind.
For us the process cost about (was a few years ago so hard to remember exactly) - all in CDN dollars:
$300 for the skills assessment
$1700 for the visa application (was a bit cheaper then)
$600 for medicals
$30 for police checks
$30 (I think) for long form birth certificates for all of us (you will need them)
$200 for certified copies and secure postage to
AustraliaSo in all it was about $3000 Canadian. This was for what would be the 175 permanent visa application (it changed after we got ours).
I should also not that on that there is only a first installment of about $2000 or so. The second installment you might see mentioned on there is only payable if you do not speak english (or your spouses) or something along those lines. So if english is your first language and you hold a Canadian passport you will only need to pay the initial application fee.
You could go for an initial consultation with an agent (usually free) that will give you a better idea of what you qualify for and what your chances are. Whether you use one or not is up to you. What I will say though is if you meet all the criteria of the skills assessment and the visa then you will get it approved. There is no judging of applications so if you've got the points, work experience, meet all the requirements (medicals, police checks, etc) then they have to give you a visa. If you are confident all those things meet the criteria then it just depends on whether you would want to do it yourself. If not, then definitely speak with an agent who can guide you along.