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700,000 doesn't seem like an insurmountable task - unless you are using phrase or title matching. These are simple searches that are better than just a default search to estimate competition, e.g.
intitle:"green widgets" [google.co.uk]
Pages mentioning the keywords (in sequence) within the title of their pages can reasonably be seen as competing. Although this doesn't take into account the many factors affecting the performance of those URLs - they could all be very poorly optimised.
Of course, next to nothing is too competitive, other than popular brand names and the like - it depends on your site, and how effective you think you are at SEO.
However, check first how many sites are already optimizing for it.
If you see that the first 50 pages have the keyword on their titles, descriptions, urls, shown on SERP, you will have a hard time SEOing, but if you see that listing are showing keywords in titles, others in descriptions, and others in urls, it means not many people have discovered that keyword for optimization, and you should start doing it.
I have been using these figures for my niches:
allintitle:blue widget
and
phrase (not broad) for the Adwords Keyword Tool, and look at the avg. monthly, not last month (which is an [exact] number)
The site I'm working on most has a Searches/allintitle ratio of 4.2; average CPC of $14; 4,400 searches per month.
It's dominated by big retailers like Walmart and big niche-specific retailers. It's all just multiple product pages so I think I can get in there by offering a little more substance and having some inbound links. (Many of the retailers' pages which show up on Page 1 have 0 inbound links.)
Sorry didn't mean to go off about my own specifics but I think we're in the same boat.