I am looking for a professional SEO firm that is not pricy but someone who knows their stuff?Any suggestions?

Study the SEO company
Read through the company website. Does the site list specific goals and techniques? Does it discuss using ethical (white hat) SEO techniques? Does it inspire confidence, or does it read like an empty sales pitch?

Check the company domain's.... How long has the domain been registered for? Does the company list complete and accurate whois details? If the domain is new, or if the whois details aren't listed, you may be looking at a fly-by-night company.

Study the SEO results
Start with the SEO company's own website. But "SEO" has 118,000,000 results in Google, and "search engine optimization" has 27,200,000 results, so don't expect the site to rank high for these generic terms. Also note that some SEO companies don't try to optimize their own sites for SEO because they get most of their business through referrals.

The important SEO results to check are those of SEO companies' clients. If they link to their clients' sites from their sites, look for keyword phrases in the title and heading tags of those sites, and see how those sites do in search results for those keyword phrases. Look for backlinks to those sites by entering "link:domain.com" (with the right domain and without the quotation marks) in different search engines.

If the SEO company is on your shortlist, contact some of those clients to ask for feedback about the SEO company. Are they still using the SEO company's services? Did they see an improvement in search results for their site, and have the improvements lasted? If they needed an SEO company for a new site, would they hire the same company?

Ask questions
The answers to these questions will show you if you're dealing with experts or people who don't really know what they're doing.

What steps do you take to optimize websites for search engines? They may not want to tell you exactly what they do, but they should list steps such as researching and using keywords, making the coding search engine friendly, and getting inbound links. If they're vague, or if they name black-hat techniques such as keyword stuffing or doorway pages, go to the next company on the list.

How do you do keyword research? They should start by learning what your site is about, what keywords already work for it, and how your business is different from the competition. A number of keyword suggestion tools are available, both free and paid. The important points are that the SEO company will research relevant, searched-for keywords for your site and that they'll look for keyword phrases, not single keywords.

Do you offer guarantees? If they offer a guarantee other than a money-back guarantee, or a promise that they won't accept payment until your site is on page X in the SERP for a specific keyword, don't trust them. The SEO business is not one in which you can guarantee exact results.

What do you think of Google's Webmaster Guidelines? If they don't stay completely within these guidelines, your site could be in trouble.

What types of sites will you try to get inbound links from? The answer should be relevant, quality sites, including directories, but not including link farms. If they don't think that inbound links are important for SEO, they aren't going to get the desired results.

Do you work on improving the conversion rate? Improving the conversion rate is not SEO. But getting more traffic to your site won't benefit you unless your site works well for site visitors after they arrive. If your conversion rate could use improvement and an SEO company can help with this area too, you benefit more. You can hire a separate company to work on your conversion rate, but they'll need to be careful not to override any of the SEO work done to your site.

How do you keep up with changes in your field? The important point here is whether they do keep up with industry news. But you'll probably know by the answers so far whether they do.

What payment structure do you use? Be wary of companies that ask you to pay the full amount at the beginning. If they're going to be doing ongoing work for you, you should be able to pay at various stages.

Expect to be questioned
The SEO company representatives should have questions for you too. They should want to find out if your expectations are realistic and fit with the type of work they do. They'll need to learn about your website, your audience, your competition, your focus, and your business goals.

Expect to wait
If the service is good, the company will be in demand, so there may be a waiting list. Start researching before you need the work to be done.