Yes but that is not a fixed cost. You pay as you go. If the dude trades 20 contracts his monthly cost is $10. Not $500. And if he is doing 1000 contracts a month his broker can and will most likely discount his rates to compensate. I don't understand why this is so confusing. If he trades nothing this month his cost is a great big zero.
That's right. And if someone is, as the OP suggests, swing trading Crude Oil calendar spreads, then paying $1 in order to execute a round turn calendar spread in order to capture, let's say, ten tics, is not an issue at all. In fact, I personally tell my own clients to jump all over the CTS-T4 or possibly the TT lease deal if they want to execute on their own.
OK, here's even harder maths. If you're doing 12,000 Round Turns per year in Nymex (your OP implies that you are trading Crude Oil Calendar Spreads) then leasing a Nymex seat for $500 per month and paying member rates would be CONSIDERABLY more cost effective than worrying about your execution platform. I'm banging on this hard because over the years I've seen traders have so many issues trying to execute spreads using free platforms and discount brokers. What works OK for day trading ES does not necessarily make it so for a spread - even an exchange supported one. And the discount broker will almost certainly not have the risk system set up to margin spreads correctly.
Deep Discount Trading. Free Platform is firetip basic ($0) or firetip pro ($49pm only). Margins are as per exchange spreads: CME http://www.cmegroup.com/clearing/margins/outright-vol-scans.html ICE https://www.theice.com/clear-europe/risk-management#energy-span-files
If you're doing a 1000 sides a month why the emphasis on the cheapest possible platform btw? Even something as simple as CQG Qtrader is relatively cheap. You mentioned you're swing trading so it's not as if the profit target is a tick.
If you take CTS with a broker like Advantage it costs $25pm without charts plus 50c per side per contract up to a maximum of $500pm. If you complete 1000 round trips per month it will cost you $500pm = $6k p/annum. http://www.advantagefutures.com/ser...plications/cunningham-trading-systems-cts-t4/