Since you already have a fat batt with 1300 mAh, about the only reasons to get a mod are to have higher (or variable) voltage or juice-feeding.
For higher voltage, rolygate is correct that
box mods are the least expensive. Here's the link for the $36
5V Ken's Box Mod (as rolygate mentioned, batteries and charger
not included). I don't think cybervex is selling the
Bartleby anymore. Another low price one is the 5V
Big KaZoo, $60 for a full kit (i.e.,
including batteries and charger).
The least expensive
variable voltage mod is the bic-lighter-sized (and shaped) $65
Little Sister.
However, I don't like box mods myself. The least expensive high voltage
tube mod is the the $53
Original (Hex) Copper (6V).
NOTE: 6V mods should only be used with "HV" (actually high
resistance) atomizers at around 4.5 ohms. Even at 5V, I recommend a semi-HV atomizer at around 3.0 ohms. These accomplish essentially the same thing as your LR atomizers on 3.4V: balance resistance and voltage to deliver optimum sweet-spot watts of power and safe amps of current. Regular resistance atomizers are good for variable voltage. And
never use LR attys at any voltage higher than 3.7V. [For more on this LR/HV, watts/amps stuff, see
this article.]
The least expensive
variable voltage tube mod is the brand new $80
Infinity.
You can get
juice-feeding by adding the $60
VapeMate or $15
The Dripper to your fat batts. [The $92 GG AFS v3.0 also shown in the photo is outside your price range and it requires a mod (and a mod-experienced user).]
The least expensive integrated juice-fed mods are the $45 side-fed
MrPuffer (shown on the left and currently unavailable) and the $50 bottom-fed
WetBox. Both use one 3.7V 900 mAh 14500 battery and a 3-ml juice bottle. Bic lighter not included.
For more information on mods (including many other choices), see my
Wonderful World of Mods in the ECF Library.