https://infinite.tcgplayer.com/article/Izzet-Delver-on-a-Budget/3a693529-192c-464c-b78d-757bdfb7e66d/

The simplest and cheapest way to improve the deck overall without making a significant change is by adding Mishra's Bauble. It works particularly well with Dragon's Rage Channeler, adding a free surveil trigger but also critically working towards Delirium with an added spell type. The budget build only plays four types, but adding this fifth type makes hitting Delirium and powering up to a 3/3 much easier. As a free spell, it's ideal to exile and unlock card advantage from Expressive Iteration. It also adds some card selection alongside the mill of Thought Scour or the shuffle effect of lands like Terramorphic Expanse putting a new card on top.

+4 Mishra's Bauble -2 Sleight of Hand -2 Counterspell

To amp up the deck's power level, you can turn to Murktide Regent. Most Modern players are familiar with the power of Gurmag Angler in these sorts of disruption-heavy decks, and Murktide Regent offers a similar threat, but with flying and the potential to be much bigger. It has emerged as one of the best cards in MH2 and a staple of Modern and Legacy, and it's a big upgrade in this deck over something like Sprite Dragon.

+4 Murktide Regent -2 Sprite Dragon -2 Dreadhorde Arcanist

If you're willing to really empty your wallet, Ragavan, Nimble Pilferer offers the deck one of the best creatures ever printed. An instant-staple in every format where it is legal, it's an incredible threat that generates both card and mana advantage, and will run away with any game where it gets going. Its power is truly unprecedented and unmatched by anything else available, and will be worth every penny if you want to be as competitive as possible in this strategy and many others. 

+4 Ragavan, Nimble Pilferer -4 Delver of Secrets

As with most decks built on a budget, there's typically a lot of room to improve the manabase with more expensive lands, and it's definitely true for this deck. Fetchlands like Scalding Tarn are a big upgrade over Terramorphic Expanse and Evolving Wilds, which are important for aiding Delirium on Dragon's Rage Channeler but have the downside of being tapped and slowing down a deck designed for speed. Compared to basic lands, Spirebluff Canal, Steam Vents, and Fiery Islet all add more consistent mana, and are great additions here that will get plenty of future use.

+4 Scalding Tarn +1 Flooded Strand +1 Misty Rainforest +4 Spirebluff Canal +3 Steam Vents +2 Fiery Islet

-4 Terramorphic Expanse -4 Evolving Wilds -4 Island -3 Mountain

The sideboard can also be bolstered, with a wide variety of alternative disruption potentially mixing up the more simple assortment in this budget build.

Blood Moon is a proven hoser, useful against a wide swath of the format.

+3 Blood Moon -2 Aether Gust -1 Abrade

Engineered Explosives is a powerful disruption spell, and while it interferes with the cheap creatures here, it's a crushing blow against the Hammer Time deck, among others.

+2 Engineered Explosives -2 Smelt

Chalice of the Void might sound weird in a deck full of one-drops, but it's perfect on 0 mana for stopping Cascade spells like Living End.

+2 Chalice of the Void -2 Spell Pierce

This strategy often uses other disruption that diversifies the sideboard with more powerful cards like Force of Negation and Subtlety. 

+1 Force of Negation +1 Subtlety -1 Mystical Dispute -1 Abrade