I believe it will be difficult to find any reliable provider that can offer 300GB of SSD space on a managed VPS. If you tone down the requirements to standard HDD space or maybe even just SSD cached/accelerated then you might be able to find something within your budget requirement.
I highly doubt you can find a 300GB SSD deal under $100/month.
Even the un-managed cheap provider like DigitalOcean's price is $320/month
I will suggest you to check accelerated / cached SSD disk plans.
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