|I figured that at after a certain number of routers a managed switch would become a more cost-effective method.|
That depends on a couple of things though.
1) Can you find one you can afford? They're not cheap and chances are you'll have to buy an older used managed switch and there's no way to know how long it will last, if it even works out of the box. It goes without saying a used switch has no warranty.
Here's an example of what you would have to deal with buying a used switch:
We have some older Cisco switches kicking around here at work. I could pick one out, default it's config, test it to ensure it's working properly. I could package it correctly but there's no guarantee it would reach you in working condition and to be honest, I wouldn't care if it didn't, it was working when I shipped it. You would have paid in advance and I would not refund your $$$ because the bill of sale would have said, " Sold to [your name] one used, as is, Cisco 2900 XL, 24 port managed switch....etc etc
So this is the risk you take buying used equipment. You might find a seller that will guarantee it and if you go that route (ie: buy a used switch) then I would recommend you only do business with someone that will guarantee it.
We don't use low-end managed switches here so I have no idea what one would cost used. I do know our 48 port, 1000 Mbps switches run around $5,000.00
2) Do you know how to configure/maintain a managed switch?
If you buy a used Cisco, it's no good to you if you don't know the Cisco CLI (command line interface) and how to properly configure it.
You could buy a used, or new, switch that isn't Cisco and comes with a nice GUI management interface, but again, you'd still have to know what you're doing and, you'd still require a router to allow you to route between subnets/VLAN's.
I'm not saying don't go that route.....personally, I would. But then I'm a network technician by trade and I'm all too familiar with VLAN tagging, subnetting, routing, and experience configuring multiple different brands of switches.
Isn't there a router I can buy for this situation which has 4 interfaces, each of which I can assign a different subnet to? Or is that just going to push my costs up way more?
Yes there are, but again, you'd have to buy new or used. I would do some pricing online. There are plenty of used network appliances for sale. I would look for a company the sells all network appliances and guarantees that at least the equipment will arrive in working order.
Again, the "do you know how to configure the device" issue comes into play. If you do, you're laughing, if you don't, then you have to learn or hire someone who does.
A SOHO Router is something you could figure out all by yourself.
Again, if you know Linux or UNIX, you could use an older PC with enough interfaces for each connection and the OS of your choice (linux/unix). You can buy multiple NIC cards......you'd need a quad (4 ports) at the least for each subnet and one more for the internet connection.
Ah yes, also: isn't a router going to find a path by broadcasting even if I don't define a route between subnets?
You have to define a route between subnets on a router in order for those subnets to speak to each other.
You have a Linux box with 5 NIC's.
eth0 = external connection that goes out to internet
eth1 = VLAN 1 - 192.168.1.0/24
eth2 = VLAN 2 - 192.168.2.0/24
eth3 = VLAN 3 - 192.168.3.0/24
eth4 = VLAN 4 - 192.168.4.0/24
VLAN 1 and VLAN 2 cannot communicate with each other as they stand since they're in completely separate subnets.
VLAN 1's broadcast address is: 192.168.1.255
VLAN 2's broadcast address is: 192.168.2.255
Both can only broadcast within their network (subnet).
In order to allow communication, you'd have to establish a static route between the two VLAN's (subnets) within the routers routing table. Essentially it would look as follows:
192.168.1.0/24 <<>> 192.168.2.0/24
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.