the 50/5 mbps bandwidth provided by our ISP. They can not offer us a higher bandwidth connection but they can offer us a second connection at the same bandwith.
With a traffic ratio like that i'm guessing you are talking about a "broadband" connection, likely cable though possiblly VDSL. Be aware that broadband connections are contended. So even if you can balance the traffic you may or may not get the speedup you desire.
Is it possible to load balance those two connections for a reasonable cost/setup?
Yes. There are three scenarios.
The best case is if your ISP is highly cooperative. You can have a single pool of IP addresses with the ISP load balancing on their end and your router load balancing on your end.
If your ISP doesn't explicity cooperate but also isn't too careful about ingress filtering you may be able to treat the IPs are one pool on your end even though the ISP is not and load-balance all outgoing traffic.
If your ISP doesn't cooperate and is doing careful ingress filtering then you will need to route traffic to the correct ISP based on it's source address. This can be done using policy routing. You can then give your webserver IP addresses from both blocks and use a DNS round robin to load balance them,
