There's a wide variety of ways to build a Sliver deck. Without turning this into a combo deck and keeping it based on combat like you seem to have it built to be...
First place I'd spend money would be the mana base. $200-300 doesn't give you room for the best mana base possible (Shocks & Fetches), but it can definitely improve it. (These suggestions for the mana base are based on how I helped my teenage son build his Sliver deck on a budget)
The problem I see with it as it stands is you're running too many basics for a 5 color deck. My Silver deck only runs one of each basic, but my other 5 color deck runs a total of 7. I think I counted 21 in your list. Personally I've never been a fan of the Vivid lands, if you have them in your opening hand or draw them early by the time you get to turn 7 or 8 they're quite possibly only tapping for one color. I'd look at the Innistrad checklands (
Glacial Fortress,
Hinterland Harbor, etc.). They can conditionally come in untapped. A full set of the 10 would probably cost you $60 (guessing, I haven't checked their prices in a few weeks). The next 10 I'd look at would be the pain lands (
Underground River,
Shivan Reef, etc.). They come into play untapped and will help speed your deck up. A full set of the 10 should probably run you around another $75-80. Now we're at 20 lands. Given your budget, you can get by the the full set of tri-lands being your next lands. That brings you to 30 lands. 7 land spots left if we're keeping the land count the same as you have it. For those last 7 spots I'd run 6 basics and a
Command Tower. The
Command Tower should be less than $2. So that's half your budget if you go to the high end of $300 but my suggestion for your mana base. You could also get away with
Sliver Hive and
Unclaimed Territory if you wanted to go up a couple lands (which I'd cut two pieces of your ramp artifacts for). There's other Rainbow lands that I could suggest but most of them are $20 or more each and would eat up the majority, if not all, of your budget.
Sliver wise, and this is just at a quick glance, the first thing that I noticed is you're running
Manaweft Sliver but not
Gemhide Sliver, same effect giving you a little redundancy.
Cloudshredder Sliver would be a good include as it gives both flying and haste.
Galerider Sliver is a nice one drop that gives all your Slivers flying as well.
Crystalline Sliver gives all your Slivers shroud.
Necrotic Sliver would give you some removal. I really didn't look at what I'd remove for these additions but if you want that advice I can give it as well.
18 pieces of artifact ramp is a lot. I'd switch to more land search based ramp spells (
Farseek,
Kodama's Reach,
Explosive Vegetation, etc.), capping it at total of 10 artifacts and spells combined as well as adding the two lands I mentioned earlier. I'd use those other spots for some more removal like
Vandalblast,
Assassin's Trophy,
Anguished Unmaking, etc.. There's more things I could suggest like adding some "cares about tribal" pieces like
Aphetto Dredging but I'd like to see where you're at with these suggestions before delving more into the deck.