I'm totally new, but I'd think the wort chiller would be great for your next purchase.
I agree, even with stove top brews the wort chiller is a godsend. It beats a sink full of ice hands down. If you get the 25 footer (which I payed 50 exactly for last year, iirc) it will fit most stove top pots, and at the same time will still cool off a 7.5 gallon turkey fryer kettle easily as well.
Just make sure your sink can take the faucet to garden hose adapter and you will be all set.
Next after that, I would look at your fermentation temp control, and like others have said, that can be as simple as a rubbemade bin and frozenplastic bottles, that's what I use. That's going to be no more than 10 dollars, so you could get more ingredients, or another couple of bucket fermenters at 12.00 apiece.
After that I WOULD do a cooler mod, because with your wort chiller and your temp control, YOU CAN do all grain on your stove top. (Or you could do Deathbrewer's way and you wouldn't need the cooler) But I prefer using a cooler mash tun, and even a 5 gallon one, (at about 30-35 for the cooler and about another 20 for parts) works great. It holds 14 pounds of grain, and in a 5 gallon batch I rarely exceed that for most of my beers. and if doing a 2.5 gallon stovetop all grain batch 14 pounds of grain is close to barelywine range.
That way, if you do even partial mashes, or stovetop all grain your cost per batch goes down considerably. If you are doing 2.5 gallon ag batches on your stove 50 bucks gets you at least 4 if not 5 2.5 gallon batches. Especially if you buy grain and hops in bulk. 50 bucks retail will get you a 50 pound sack of 2-row which is a lot of beer, especially if you harvest yeast, and toast your own grain to make your various crystal malts.
And if you can get into a bulk grain buy, that will be even less, the last couple buys the Michigan Mashers did, the 50 pound bag price was down to iir 35 bucks...so again, 15 bucks more to spend on other stuff.
If you can reduce, if not cut out your extract usage per batch, your costs will go down. Even doing a partial mash with half your fermentables coming from grain rather than extract will make your 50.00 budget go far.
Next after that I would look at moving outside to a turkey fryer and full volume 5 gallon boils. If you hunt around you can pick them up off season (like even right now since the holidays are over) Pretty cheap, I paid 25.00 for mine at a grocery store. So again if you find them for 25, that's another 25 for ingredients or another fermenter or something else, like bulk hops, or some other malts.
The 7.5 gallon kettle is fine for just starting out, heck I have been using mine for 3 years now and it works fine. I haven't yet upgraded to a keggle.
Though a bottle of fermcap-s foam control is a must for both a turkeyfryer kettle or stovetop brewing setup. In a 5 gallon stove top pot I have been able to have a stove top rolling boil about 1" below the rim of the kettle and gone and napped between hop additions without fear of ruining the stovetop.
Somewhere in this mix, I would buy some brewing software, if you don't already have some some. Free one's like beercalculus are awesome, but limited, and beersmith at 25 bucks has an awesome amount of features. And again, 25 for that means 25 for something else.
After that although not necessary, but really helpful is a refractomter, the nice thing about it is that it uses very little wort to take you initial gravity readings, and that is invaluable for small batch brewing. And maybe Forrest at Austin Hombrew will have another great 25.00 refractometer sale, but you can get them on ebay.
If you did that, spread out over 6 months or a year, your 50 bucks a month budget will go far. After you get that basic setup then your monthly budget will go for ingredients.
To me kegging, on your budget should be a low priority. Personally it's better to get your brewing process nailed down, and get your
batch cost, in regards to ingredients, to stretch far.
And bottles are pretty free. And if you nail your bottling process down, it's really no more effort than kegging. And a lot cheaper. You don't need something to keep your kegs cold, you don't need kegging equipment.
To be that's a luxury, and at 50.00/month brewing budget, you need to concentrate on necessities.
It's the same with fermenters, better bottles and glass carboys are not cheap, buckets are. And if like many of us you opt not to secondary for the majority of your beers, then having more primaries is more important than any vessel to secondary in. And even if you brewe a lot, and secondary to add fruit or oak, you maybe only need one dedicated secondary vessel. I have 9 primaries, and - 5 gallon glass carboy I rarely use. Because I rarely secondary.
Hope this helps. I may be a little more pragmatic that the other folks, but I think over a few months you can do awesomely with a 50 dollar budget.