this one's tricky. If the seller knows what they're doing there's a reason why they've priced link insertions so cheaply, and they obviously won't tell you the reason.
It's possible that your link will be inserted into a page that's penalized by google in some way, or it's completely deindexed (in which case your link insertion will have zero value).
It's also possible that the link insertion is on a page with dozens of other links, in which case the value you'll get out of this link will be minimal at best, or - most likely - zero at worst.
Not sure that this is true, but there's a myth going on around the SEO community that google rewards the 1st link on a page with the most authority (link juice) while all subsequent links on the page only getting a fraction of the juice.
Again, this might be a myth only, but if it's true and your link insertion is 10th or twentiest on the page... well, no wonder the price is so low in that case as you'll hardly get any value out of it
On the other hand, the guest post - while mostly useless in the beginning due to being placed on a page with zero authority and zero traffic - might be worth its price in gold in 6-12 months when the guest post ages, ranks, and starts getting traffic and authority, both of which will trickle down into your website...
You really need to do your due diligence on these things, as SEO is one of the most smoke and mirrors industry for newbies, it's so easy to be scammed by seasoned experts that know what they're doing and how to push your buttons to give them money...