SEO is a tough one to hire for.
The reason being that there's so much noise out there, and so many who say they can do it, ranging from cheap outsourced people on Upwork to big New York City ad agencies who claim it's something they'll through into your contract with them.
What also make it tough is that yes, it's generally true that it takes 4-12 months to really see organic traffic results from SEO. Google itself says this: https://www.youtube.com/watch?v=piSvFxV_M04
So when you hire someone, even if they're doing quality work, it can be 4 months before you know for sure, and you've already spent that money.
If you are on a really tight budget, you have to either learn the basics yourself so that you can filter out the cruft, or rely on a trusted friend who used someone and can prove the improvements through Google Analytics reports.
To learn SEO, you should read sites like Moz.com and Backlinko.com, but you also need to get in the weeds and do it yourself - at least for a few months.
As far as a reasonable price, it all comes down to ROI. It's truly a difficult question to answer. I've thought about this a lot, and I think the best way to do it is to measure the value you'll get by ranking just your most important page for just your most important keywords. Focus on that small, narrow page first before expanding to your whole site. The concept here is that you may find that, hey if we get to the #3 ranking for these 10 keywords for this page, that will bring in 2,000 visits a month, which at a 3% conversion rate is worth $24,000 a month for our business. (Plug in your own numbers)
This epic post and ebook and template from Siege Media perfectly illustrates that -- http://www.siegemedia.com/seo/seo-roi-from-content-marketing
On average, a USA based SEO agency won't offer services for less than $1,500 a month. I would say the average reputable agency or consultant would charge $2,500 - 15,000/month for their services. But this can go all the way up to $100k+/month if you were a globo corp.
For a startup, you may want to look a solid smaller agencies or consultants or look outside the USA. The problem with finding cheaper labor on Upwork, especially for SEO, is that the results can be negative or very marginal. So does it matter that it's only $800/mo if it produces almost nothing, when a $2,500/mo service produces 50x more?
You won't know how to evaluate unless you learn the basics and run the numbers.
The best thing you as a business owner can do is be very clear up front what your best products or services are, what you need to get to break even, and what your ideal timeline is. If you're not clear on your average order value, customer lifetime value, or profit margins, you can't expect the SEO to figure that you, and if nobody knows these things, then all the efforts are not tied to business metrics, which makes things complex and without a solid strategy.