Marauder Setup cost

 Make sure you have a place to get the tank filled before you buy one! 
A used 3300 psi scuba tank is probably the least expensive and most useful fill setup IMO, they're not common but are around. Next best would be used 3000 psi tank. There's a guy in my area who does Hydro testing and he always has some tanks for sale. Mine cost me $125 each. You'll also need a K-valve fill setup, they're available lots of places, but I'd recommend going for the best quality and get one from Joe at airtanksforsale.com. His fill whip has a bigger gauge and a microbore hose. Just checked his site, the fill setup is $139.
Everyone will recommend the .25 cal. Marauder, and they are great shooters, BUT they are airhogs, only getting 16 shots on the stock tune, And needing a 3000 psi fill pressure. My advice would be to find a used .177 or .22 with a proven good barrel and go from there, they usually come tuned to work at a 2500 psi fill pressure. It is possible to tune any Marauder for use with a lower pressure fill, BUT you'll need a chronometer, lots of air, pellets, time and patience. That being said I have two Marauders, .22 and .25 that work on lower pressure fills and don't think I could get much better accuracy and performance without jumping to a carbon fiber tank ($$$!) and very expensive PCP rifles. Another rifle to consider is the Air Arms CZ S200 hunter, it works on a 2600 psi fill and is said to be very accurate, though not as powerful or quiet as the Marauder, and it's a bit more expensive.
Good luck,
John

Check to make sure that there's a place to fill an SCBA tank first. This is a good place to start.
http://www.crosman.com/discover/airguns/pcp-resources/find-a-fill-station

You might need to search around for paintball and/or Scuba dive shops. Some people get them filled at the local fire station. Make sure that whoever you talk to can do 4500PSI tanks. Many places will only do 3000psi for paintball/dive tanks.

Then decide if you want to buy new or build your own tank setup. I documented my build project with links to all of my equipment here if you're interested:
http://airgunnation.dev/topic/ebay-air-tank-project-is-complete/
Some of the links might be out dated now, but the important ones are still active. The guy selling the tanks still has more than 250 of the tanks available, so no rush there. They are used tanks made in 2008 so they'll need to be tested again. You'll need to see if there's a local place that will test them for you. If not, you can send them to hydrotester (http://hydrotester.com/) in Bremerton WA. There might be other places that do it by mail, but that's the only place I know of.

If you go for a bare-bones set up of tank, whip, and gauge assembly, you can do it for about $300 depending on what options are available locally.

66 ft^3 4500psi SCBA tank: $150
Hydrotest: $25
Fill Wip: $20-$50
Gauge Assembly: $75-$150

You might also consider getting a Marauder Pistol. I think it works better as a rifle than a pistol. As a bonus, it's easy to pump by hand.

Check this out:
For $450 you'd get everything you need to start shooting. 
http://www.gatewaytoairguns.org/GTA/index.php?topic=103333.msg975080#msg975080

Edit: Just realized you already have a pump. You could probably sell one of the pumps to offset the purchase price of the gun a bit.

I have scuba shops by me and just got a scuba tank 3000psi for $89. I get lots of fills on my .177 & .22 Mrod tuned by me about
30 shots per fill 2600 psi

It is 2016, NO exercise or effort is liked too well by many. That said, you've pumped your discovery so you will have no problem pumping a marauder, any healthy person can use a hand pump and even going to 3,000psi isn't very hard.

A 3,000psi tank will work as you'll be tuning ( adjusting but folks like to say "tuning" ) it to suit you anyway and very likely will not be going to a full 3,000psi unless you have a true need for max power. As stated scuba tank are fairly low cost and if you find a steel tank ( they do cost a bit more) it should have a 3442psi fill, and most are larger. I have a110cf 3442 steel tank and it is a good bit of air,

You will enjoy the benefits of a marauder over a discovery so much you'll not even notice pumping because your smilling so much from the huge step up in airguns/

John