For Relentless Flame from just the contents of two theme decks:
Pokemon (23):
4 Charmander
4 Charmeleon
4 Charizard
3 Nidoran
3 Nidorina
3 Nidoqueen
2 Farfetch'd
Trainers (20):
4 Cynthia
4 Copycat
3 Nest Ball
3 Pokemon Fan Club
3 Brock's Grit
2 Professor Kukui
1 Switch
Energy (16):
16 Fire Energy
Tweaks:
-2 Kukui
+2 Choice Band (TCGPlayer market price of $0.55 each)
-4 Copycat
+4 Lillie (Shining Legends copy has a market price of $0.25 each)
-2 Charmeleon
-1 Nidorina
-1 Nest Ball
+4 Rare Candy (Primal Clash copy has a market price of $1.07 each)
-2 Farfetch'd
+2 Sun & Moon Oranguru (the one with Instruct; $2.92 Market Price) Oranguru might push you over the $30 limit when combined with the deck and the other purchases, but should still be fairly close to $30.
That's a standard legal deck until Rotation hits on August 15th. Maybe drop the Switch for a Guzma ($2.86 Market Price) and a Fan Club for a Tapu Lele GX (alternate art has market price of $9.94 atm) if you're feeling adventurous.
For Lost March, here's my list (price is each):
4 Hoppip LOT ($0.10 for 30 HP/$0.20 for 40 HP)
4 Skiploom LOT ($0.25)
4 Jumpluff LOT ($1.24 for regular, $0.43 for league promo)
4 Trumbeak LOT ($0.30)
3 Natu LOT ($0.16)
3 Emolga TEU ($0.11)
1 Marshadow SLG ($3.92)
1 Oranguru SM ($2.92)
4 Lillie ($0.25)
4 Cynthia ($2.61)
4 Net Ball ($0.79)
4 Great Ball ($0.15 in Shining Legends)
3 Lost Blender ($0.23)
3 Pokemon Communication ($0.30 for Black & White Base Set one, but it may be easier to find the Team Up print which is $0.33)
2 Choice Band ($0.55)
1 Guzma ($2.86)
1 Nest Ball ($0.55)
5 Grass Energy ($0.10)
4 Double Colorless Energy ($0.54 for Fates Collide print)
Which gives us a market price floor of $35.93 for Lost March. Marshadow isn't necessary but helps as both disruption and draw support. Fairly cheap overall given most things wanna run Dedenne GX or Tapu Lele GX right now.