First Deck Build Attempt (Dimir)

Kaffieen's comments are pretty spot on. You might give us some more focus on which way you'd like to go.

Also, if you're going Dimir, don't mess with Ravnica fatpacks.

You actually want to look at pack prices specifically, in buying cards. Fatpacks usually are the worst way to buy cards.
Consider this, a fatpack has 9 booster packs (and some lands, dice, etc).
Starcitygames (which is usually more expensive than most major card shops) sells gatecrash packs at 3$/per. So that's $27, to get those same 9 booster packs. You won't typically find fatpacks for less than ~$38 in a store. That means you're over paying bigtime buying fatpacks.

Don't buy boosters at walmart, you're almost always better off going to a game shop to buy them. Walmart charges $4 a booster. Card shops are usually 3.

There *are* ways to get cards for cheaper. I compete locally (in the south east), and my local card shop gives prizes in booster packs, boxes, and fatpacks. I generally sell fatpacks *all* the time. My post on that is here: http://deckbox.org/forum/viewtopic.php?id=6532

You can do the math, but if you're buying 2 or more, I can get them to you cheaper than *anywhere* else can

On to your deck. With direction, we can help you focus it a bit more. I can tell you Dimir is a very solid middle tier deck. Your local card shop probably has FNM (unless you live in the boonies, in which case you'll need to use wizard's site to find a place). They will have prizes for FNM. Different shops do it differently. Some charge some money to play in FNM, some don't. Free places typically don't have much prizes. If you're paying to play, you should get *at least* a booster ever time. Doesn't mean its the best way to get cards, but it's A way.

If you do well, they have prizes for the higher win-ratio players.

I will tell you, you can get a good dimir deck for a *lot* less money than a good Boros deck, or something else. However, the reason is: Dimir isn't as good. Plain and simple. I'm not saying it's bad, but other color combonations will win more frequently. Those decks are also going to be 2-5x as much money to build. It just all depends on what you're able to do.

I would absolutely make use of this site, and of cockatrice (google it) to learn what you want your deck to do FIRST. BEFORE investing in going a certain way.

If you're looking into Sage's Row Denizen, I would suggest looking at running 4x Faerie Impostor.  If you have two (one in play and one in hand) with the Denizen out, they turn into a mill engine of their own "U: cast Impostor, unsummon impostor, opponent mills two."  It's an easy to disrupt circle, but it could be fun if you ever get it off the ground (toss an aberration onto the field and you're milling for a LOT).

From a competitive standpoint, milling is among the hardest things to do - given that you're making your job three times harder than it is originally.  It's a ton of fun if you get it to work, but it's unlikely and is particularly frustrating as you'll end up losing more games than not.

In terms of making a moderately competitive build, depending on the format, costs can range pretty wildly.  I last saw someone rocking a pretty cheap enchantment build using Ethereal Armor and mostly commons/uncommons.  If you were to go G/W you can likely put something together that would put the pressure on (and win matches here and there) for somewhere around $50-100, even cheaper if you're not worried about the dual lands.

Sealed deck is often times the best way to go, but it can be a bummer to never play with the cards you've been collecting.

Good luck whichever way you decide do take things, and remember, we're here to help with any ideas you might have.