I have consulted with a few companies that have had similar issues to this, I´m gonna make a few assumptions here in order to answer your question.
Based on your product cost and your annual revenue I would say you have at least 66 customers who have purchased your product.
A quick solution to grow revenue in the meantime that will help increase revenue and keep your inside sales guy busy (you mentioned he still has time on his hands after getting through all of your leads) would be to go back to these 66 or so clients and upsell/cross sell to them if you have other products or sell your current product into other areas of the businesses who have already bought from you.
Even if you don´t pick up more revenue from these customers because they don´t need anything else from you then you can maybe get professional referrals into other customers in different businesses. It´s an extra phone call for your inside sales guy to make but it´s definitely low hanging fruit and no additional investment.
To answer your main question, the way to figure it out is to look at your sales funnel and see what the conversion rates are from the leads you are getting through to close.
If you are getting a low conversion from inbound lead to qualified opportunity, you simply may need to tweak either the inbound lead source or the data that you capture online in order to improve the quality of the inbound leads.
Based on your quoted revenue and product cost I have calculated that your inside sales guy closes 6 deals a month. Estimating that you get 50-100 leads a month, that gives you a conversion of between 6-12% which is pretty nice. But that still means 88-94% of deals fall out of the funnel.
If you look at where most of your leads fall out of the sales funnel it will tell you what tweaks your online marketing requires for example if the data has incorrect contact info or the prospects are confused about why you're contacting them.
This way you can improve the quality of the leads meaning your inside sales guy will spend more time having sales conversations and less time qualifying leads out. When you go to increase the leads you give to the sales guy you will also be giving him quality. If you increase the lead volume too quickly or with new suppliers you run the risk of decreasing the quality of the leads and making the inside sales rep less likely to follow them up.
On the other hand with a BDM you know a smaller quantity of high quality leads will get passed to the sales guy (ensure to base their commission on closed or advanced sales stage deals to ensure they drive quality.)
Hiring a BDM with a low/no salary raises a few questions;
- What data are they going to call on?
- What level of experience will they have?
- Who follows up the leads that the BDM generates?
- How will their commission be split, will it depend on the IS guy to close it?
To sum up, the best option would be to increase the lead volume but keep a strict eye on quality. If your additional lead volume manages to help you increase revenue to hit your 1.5mm target it will always be the most cost effective.
Even if you hire the best BDM in the world and he agrees to work for commission only you are always going to spend more on his commission per lead than an inbound marketing lead.
I would be happy to review in more detail with you if you have questions.