Why is PLGA so expensive in comparison to PLA?
Poly(lactic-co-glycolic acid) or PLGA is a biodegradable copolymer of lactic acid ($\ce{CH3CH(OH)CO2H}$) and glycolic acid ($\ce{HOCH2CO2H}$). Poly(lactic acid) or PLA is, on the other hand, a polymer of lactic acid, which is also biodegradable. Since both polymers are linked together by ester linkages of their monomers, the resulting polymers can be categorized as linear, aliphatic polyesters:

PLGA is one of the most successfully biodegradable polymers, and hence it attracts considerable attention in the development of drug delivery systems due to its desirable properties and to its Food and Drug Administration (FDA) and European Medicine Agency (EMA) approval for parenteral administration. PLGA encapsulated drug delivery enjoys drug protection from degradation. In addition, PLGA also provides prolonged controlled release of encapsulated drugs. Therefore, as you have suspected, these medical uses (current and developing applications) needed GMP manufacturing of the product for the purpose of the use in Humans. This GMP manufacturing of drugs is always costly. The price of the compound also depend on the other factors such as the size of the polymer (molecular weight), percent enantiomeric ratio (Lactic acid has two enanthiomers), end groups, etc., which are critical in human use. Also remember, synthesis of copolymer with desired ratio is not a easy task.
PLA has become a popular material due to it being economically produced from renewable resources. In 2010, PLA had the second highest consumption volume of any bioplastic of the world. For example, PLA is the most widely used plastic filament material in 3D printing. This commercial success of PLA makes its bulk production (no need of GMP manufacturing processes). Thus, it is much cheaper than demanding PLGA. Yet, PLA can be expensive as well, depending on its use, which require careful synthetic processing. For example, D- and L-lactide copolymer is used in bone engineering (Ref.1).
Sigma-Aldrich listed series of PLGA and PLA with some different properties:
$$
\begin{array}{c|ccc}
\hline
\text{EXPANSORB$^®$} &amp; \text{PLGA or PLA} &amp; \text{L/DL Ratio} &amp; \text{End group} &amp; \text{Molecular weight} &amp; \text{Price}/\pu{10 g}\\
\hline
\text{DL 100-2A} &amp; \text{PDLLA} &amp; 0/100 &amp; \ce{COOH} &amp;  \pu{10-25 kD} &amp; \pu{501.00 USD}\\
\text{DLG 50-2A} &amp; \text{PLGA} &amp; 50/50^a &amp; \ce{COOH} &amp;  \pu{5-20 kD} &amp; \pu{446.00 USD} \\
\text{DLG 75-9E} &amp; \text{PLGA} &amp; 75/25^b &amp; \text{Ester} &amp;  \pu{100-150 kD} &amp; \pu{458.00 USD} \\
\text{DLG 75-2A} &amp; \text{PLGA} &amp; 75/25^b &amp; \ce{COOH} &amp;  \pu{5-20 kD} &amp; \pu{446.00 USD} \\
\hline
\end{array}\\
^a: \text{D/L-Lactic : Glycolic} = 50:50; \ ^b: \text{D/L-Lactic : Glycolic} = 75:25
$$
The list was posted to show when synthesized for biological usage, either polymer can be expensive. For example, the entry 1 of the above table is essentially PLA (made by 100% D/L-lactic acid), but it is the most expensive one on the list.
References:

C. X. F. Lam, R. Olkowski, W. Swieszkowski, K. C. Tan, I. Gibson, D. W. Hutmacher, &quot;Mechanical and in vitro evaluations of composite PLDLLA/TCP scaffolds for bone engineering,&quot; Virtual and Physical Prototyping 2008, 2(4), 193-197 (https://doi.org/10.1080/17452750802551298).